feat(payments): add the payments schema, currency domain and money type

Stand up the payments data foundation: a self-contained `payments` Postgres
schema for the in-game currency, wallets, benefits, catalog, orders and the
append-only operations ledger, behind a domain package — nothing wired to real
money yet.

- Migration 00010: the `payments` schema and a NOLOGIN confinement role (ALL on
  payments.*, nothing on backend); the ledger with a BEFORE UPDATE/DELETE
  append-only trigger and a partial idempotency index; the materialised
  balances/benefits; the catalog (atoms seeded) + products + per-method prices;
  the typed single-row config; orders and payment_events. There is no
  cross-schema foreign key — account_id is a plain uuid kept consistent in code,
  which keeps the domain extractable. Expand-contract and reversible.
- Money is a bigint in the currency's minor units carried by a `Money` value
  type (exact, math/big): no float ever touches an amount, and a whole-unit
  currency cannot hold a fraction.
- Extend jetgen to generate the payments schema; construct the service in the
  composition root behind a narrow interface with a boot reachability check.
- Tests: integration (role confinement via SET ROLE, the append-only trigger,
  CHECK constraints, the idempotency index, and a forward+backward migration),
  Money unit tests, and an import-boundary test keeping the payments jet code
  private to the domain.
- Docs: PLAN.md, docs/PAYMENTS.md (+ _ru mirror) updated to the built model.

backend/internal/payments/boundary_test.go

@@ -0,0 +1,57 @@
+package payments
+
+import (
+	"go/parser"
+	"go/token"
+	"io/fs"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+// TestPaymentsSchemaImportBoundary enforces that only this package reaches the
+// payments jet code. The payments schema is isolated behind this domain; the
+// application connects to Postgres as a superuser that bypasses the schema
+// grants, so the runtime wall that keeps every other package out of payments.*
+// is this import boundary, not the DB privileges. If another package imported
+// the generated payments tables it could issue SQL against the schema directly,
+// breaking the single-writer guarantee and the domain's extractability.
+func TestPaymentsSchemaImportBoundary(t *testing.T) {
+	const jetPkg = "scrabble/backend/internal/postgres/jet/payments"
+
+	_, thisFile, _, ok := runtime.Caller(0)
+	if !ok {
+		t.Fatal("cannot resolve the test file path")
+	}
+	// thisFile = .../backend/internal/payments/boundary_test.go
+	backendRoot := filepath.Clean(filepath.Join(filepath.Dir(thisFile), "..", ".."))
+	allowedPrefix := filepath.Join(backendRoot, "internal", "payments") + string(filepath.Separator)
+
+	fset := token.NewFileSet()
+	walkErr := filepath.WalkDir(backendRoot, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() || !strings.HasSuffix(path, ".go") {
+			return nil
+		}
+		file, perr := parser.ParseFile(fset, path, nil, parser.ImportsOnly)
+		if perr != nil {
+			return perr
+		}
+		for _, imp := range file.Imports {
+			p := strings.Trim(imp.Path.Value, `"`)
+			if p != jetPkg && !strings.HasPrefix(p, jetPkg+"/") {
+				continue
+			}
+			if !strings.HasPrefix(path, allowedPrefix) {
+				t.Errorf("%s imports %s — only internal/payments may reach the payments jet code", path, p)
+			}
+		}
+		return nil
+	})
+	if walkErr != nil {
+		t.Fatalf("walk backend tree: %v", walkErr)
+	}
+}

backend/internal/payments/money_test.go

@@ -0,0 +1,136 @@
+package payments
+
+import "testing"
+
+func TestMoneyFromMinor(t *testing.T) {
+	m, err := MoneyFromMinor(14950, CurrencyRUB)
+	if err != nil {
+		t.Fatalf("MoneyFromMinor: %v", err)
+	}
+	if m.Minor() != 14950 || m.Currency() != CurrencyRUB {
+		t.Fatalf("got minor=%d currency=%s", m.Minor(), m.Currency())
+	}
+	if _, err := MoneyFromMinor(1, Currency("EUR")); err == nil {
+		t.Fatal("expected an error for an unknown currency")
+	}
+}
+
+func TestMoneyFromMajor(t *testing.T) {
+	cases := []struct {
+		major int64
+		cur   Currency
+		minor int64
+	}{
+		{149, CurrencyRUB, 14900}, // roubles scale to kopecks
+		{250, CurrencyStar, 250},  // Stars are whole units
+		{7, CurrencyVote, 7},
+		{50, CurrencyChip, 50},
+	}
+	for _, c := range cases {
+		m, err := MoneyFromMajor(c.major, c.cur)
+		if err != nil {
+			t.Fatalf("MoneyFromMajor(%d,%s): %v", c.major, c.cur, err)
+		}
+		if m.Minor() != c.minor {
+			t.Errorf("MoneyFromMajor(%d,%s): minor=%d, want %d", c.major, c.cur, m.Minor(), c.minor)
+		}
+	}
+	if _, err := MoneyFromMajor(1<<62, CurrencyRUB); err == nil {
+		t.Fatal("expected an overflow error")
+	}
+}
+
+func TestParseMoneyExact(t *testing.T) {
+	cases := []struct {
+		text  string
+		cur   Currency
+		minor int64
+	}{
+		{"149.50", CurrencyRUB, 14950},
+		{"149", CurrencyRUB, 14900},
+		{"0.01", CurrencyRUB, 1},
+		{"0.10", CurrencyRUB, 10}, // 0.1 is inexact as a float; big.Rat keeps it exact
+		{"-5.00", CurrencyRUB, -500},
+		{"250", CurrencyStar, 250},
+		{"7", CurrencyVote, 7},
+		{"50", CurrencyChip, 50},
+	}
+	for _, c := range cases {
+		m, err := ParseMoney(c.text, c.cur)
+		if err != nil {
+			t.Fatalf("ParseMoney(%q,%s): %v", c.text, c.cur, err)
+		}
+		if m.Minor() != c.minor {
+			t.Errorf("ParseMoney(%q,%s): minor=%d, want %d", c.text, c.cur, m.Minor(), c.minor)
+		}
+	}
+}
+
+func TestParseMoneyRejectsFraction(t *testing.T) {
+	// A whole-unit currency (Vote/Star/chip) must never accept a fraction, and
+	// the rouble must never accept finer than a kopeck — the no-float gate.
+	bad := []struct {
+		text string
+		cur  Currency
+	}{
+		{"250.5", CurrencyStar},
+		{"1.5", CurrencyChip},
+		{"7.01", CurrencyVote},
+		{"1.999", CurrencyRUB},
+		{"0.001", CurrencyRUB},
+		{"abc", CurrencyRUB},
+		{"12.34", Currency("EUR")}, // unknown currency
+	}
+	for _, c := range bad {
+		if m, err := ParseMoney(c.text, c.cur); err == nil {
+			t.Errorf("ParseMoney(%q,%s) = %d, want an error", c.text, c.cur, m.Minor())
+		}
+	}
+}
+
+func TestMoneyAddCmp(t *testing.T) {
+	a, _ := MoneyFromMinor(14900, CurrencyRUB)
+	b, _ := MoneyFromMinor(50, CurrencyRUB)
+	sum, err := a.Add(b)
+	if err != nil || sum.Minor() != 14950 {
+		t.Fatalf("Add: minor=%d err=%v", sum.Minor(), err)
+	}
+	if c, err := a.Cmp(b); err != nil || c != 1 {
+		t.Fatalf("Cmp a>b: got %d err=%v", c, err)
+	}
+	if c, err := b.Cmp(a); err != nil || c != -1 {
+		t.Fatalf("Cmp b<a: got %d err=%v", c, err)
+	}
+	if c, err := a.Cmp(a); err != nil || c != 0 {
+		t.Fatalf("Cmp a==a: got %d err=%v", c, err)
+	}
+	// Cross-currency arithmetic is refused, not silently coerced.
+	star, _ := MoneyFromMinor(1, CurrencyStar)
+	if _, err := a.Add(star); err == nil {
+		t.Error("Add across currencies: expected an error")
+	}
+	if _, err := a.Cmp(star); err == nil {
+		t.Error("Cmp across currencies: expected an error")
+	}
+}
+
+func TestMoneyString(t *testing.T) {
+	cases := []struct {
+		minor int64
+		cur   Currency
+		want  string
+	}{
+		{14900, CurrencyRUB, "149.00 RUB"},
+		{14950, CurrencyRUB, "149.50 RUB"},
+		{1, CurrencyRUB, "0.01 RUB"},
+		{-500, CurrencyRUB, "-5.00 RUB"},
+		{250, CurrencyStar, "250 XTR"},
+		{5, CurrencyChip, "5 CHIP"},
+	}
+	for _, c := range cases {
+		m, _ := MoneyFromMinor(c.minor, c.cur)
+		if got := m.String(); got != c.want {
+			t.Errorf("Money{%d,%s}.String() = %q, want %q", c.minor, c.cur, got, c.want)
+		}
+	}
+}

backend/internal/payments/payments.go

@@ -0,0 +1,171 @@
+// Package payments is the in-game currency, wallet, benefit and catalog domain.
+//
+// It owns its own Postgres schema (payments) and is the only backend package
+// that issues SQL against it — an import-boundary test forbids any other package
+// from importing the payments jet code, which keeps the domain extractable into
+// its own database or process later. There is no cross-schema foreign key to the
+// account schema: an account id is a plain uuid here, kept referentially honest
+// in code.
+//
+// Money is carried exclusively by [Money] — an exact integer amount in a
+// currency's minor units — so no floating-point value ever reaches a monetary
+// amount, and a whole-unit currency (Vote, Star, chip) can never hold a
+// fraction. This file is the data-foundation layer: the currency value type; the
+// wallet mechanics build on the schema and this package later.
+package payments
+
+import (
+	"fmt"
+	"math/big"
+)
+
+// Currency identifies the unit a monetary amount is denominated in.
+type Currency string
+
+const (
+	// CurrencyRUB is the Russian rouble; its minor unit is the kopeck (1/100).
+	CurrencyRUB Currency = "RUB"
+	// CurrencyVote is the VK Vote — a whole unit with no sub-unit.
+	CurrencyVote Currency = "VOTE"
+	// CurrencyStar is the Telegram Star (XTR) — a whole unit with no sub-unit.
+	CurrencyStar Currency = "XTR"
+	// CurrencyChip is the in-game chip, the unit a value's price is quoted in —
+	// a whole unit with no sub-unit.
+	CurrencyChip Currency = "CHIP"
+)
+
+// minorPerUnit reports how many minor units make one major unit of the currency.
+// Every currency except the rouble is a whole-unit currency (scale 1), so an
+// amount in it structurally cannot carry a fraction.
+func (c Currency) minorPerUnit() int64 {
+	if c == CurrencyRUB {
+		return 100
+	}
+	return 1
+}
+
+// Valid reports whether the currency is one of the known units.
+func (c Currency) Valid() bool {
+	switch c {
+	case CurrencyRUB, CurrencyVote, CurrencyStar, CurrencyChip:
+		return true
+	default:
+		return false
+	}
+}
+
+// Money is an exact monetary amount: a signed integer count of a currency's
+// minor units (rouble kopecks; Vote/Star/chip whole units). It is the sole
+// carrier of money in the payments domain — construction, arithmetic and
+// formatting all go through it, so no float ever reaches an amount and a
+// whole-unit currency can never hold a fraction. The zero value has an empty,
+// invalid currency; build a value with [MoneyFromMinor], [MoneyFromMajor] or
+// [ParseMoney].
+type Money struct {
+	minor    int64
+	currency Currency
+}
+
+// MoneyFromMinor builds a Money from a raw count of the currency's minor units
+// (kopecks for the rouble, whole units otherwise). It errors on an unknown
+// currency.
+func MoneyFromMinor(minor int64, c Currency) (Money, error) {
+	if !c.Valid() {
+		return Money{}, fmt.Errorf("payments: unknown currency %q", c)
+	}
+	return Money{minor: minor, currency: c}, nil
+}
+
+// MoneyFromMajor builds a Money from a whole number of major units (roubles,
+// Votes, Stars, chips). It errors on an unknown currency or on overflow.
+func MoneyFromMajor(major int64, c Currency) (Money, error) {
+	if !c.Valid() {
+		return Money{}, fmt.Errorf("payments: unknown currency %q", c)
+	}
+	scaled := new(big.Int).Mul(big.NewInt(major), big.NewInt(c.minorPerUnit()))
+	if !scaled.IsInt64() {
+		return Money{}, fmt.Errorf("payments: %d %s overflows", major, c)
+	}
+	return Money{minor: scaled.Int64(), currency: c}, nil
+}
+
+// ParseMoney parses a decimal amount (e.g. "149.50", "250") in the currency,
+// exactly and without floating point (via math/big). It rejects a value with
+// finer precision than the currency allows — in particular, any fractional part
+// for a whole-unit currency — which is the gate that keeps a fraction out of an
+// integer currency.
+func ParseMoney(text string, c Currency) (Money, error) {
+	if !c.Valid() {
+		return Money{}, fmt.Errorf("payments: unknown currency %q", c)
+	}
+	r, ok := new(big.Rat).SetString(text)
+	if !ok {
+		return Money{}, fmt.Errorf("payments: %q is not a valid amount", text)
+	}
+	scaled := new(big.Rat).Mul(r, new(big.Rat).SetInt64(c.minorPerUnit()))
+	if !scaled.IsInt() {
+		return Money{}, fmt.Errorf("payments: %q has finer precision than %s allows", text, c)
+	}
+	num := scaled.Num() // the denominator is 1 once scaled to an integer
+	if !num.IsInt64() {
+		return Money{}, fmt.Errorf("payments: %q overflows %s", text, c)
+	}
+	return Money{minor: num.Int64(), currency: c}, nil
+}
+
+// Minor returns the amount as a count of the currency's minor units — the value
+// persisted to an amount column.
+func (m Money) Minor() int64 { return m.minor }
+
+// Currency returns the amount's currency.
+func (m Money) Currency() Currency { return m.currency }
+
+// IsZero reports whether the amount is zero.
+func (m Money) IsZero() bool { return m.minor == 0 }
+
+// Add returns the sum of the two amounts. It errors when the currencies differ.
+func (m Money) Add(o Money) (Money, error) {
+	if m.currency != o.currency {
+		return Money{}, fmt.Errorf("payments: cannot add %s to %s", o.currency, m.currency)
+	}
+	return Money{minor: m.minor + o.minor, currency: m.currency}, nil
+}
+
+// Cmp compares the two amounts, returning -1, 0 or +1. It errors when the
+// currencies differ.
+func (m Money) Cmp(o Money) (int, error) {
+	if m.currency != o.currency {
+		return 0, fmt.Errorf("payments: cannot compare %s to %s", o.currency, m.currency)
+	}
+	switch {
+	case m.minor < o.minor:
+		return -1, nil
+	case m.minor > o.minor:
+		return 1, nil
+	default:
+		return 0, nil
+	}
+}
+
+// String renders the amount as "<value> <currency>", with the currency's
+// fractional digits and no floating point (e.g. "149.50 RUB", "250 XTR").
+func (m Money) String() string {
+	scale := m.currency.minorPerUnit()
+	if scale == 1 {
+		return fmt.Sprintf("%d %s", m.minor, m.currency)
+	}
+	neg := m.minor < 0
+	abs := m.minor
+	if neg {
+		abs = -abs
+	}
+	width := 0
+	for s := scale; s > 1; s /= 10 {
+		width++
+	}
+	sign := ""
+	if neg {
+		sign = "-"
+	}
+	return fmt.Sprintf("%s%d.%0*d %s", sign, abs/scale, width, abs%scale, m.currency)
+}

backend/internal/payments/service.go

@@ -0,0 +1,17 @@
+package payments
+
+import "context"
+
+// Service is the payments domain's application layer — the narrow surface other
+// domains depend on, keeping the schema reachable through one seam. The
+// data-foundation layer exposes only a health check; the wallet, benefit and
+// store-compliance operations arrive with the currency mechanics.
+type Service struct {
+	store *Store
+}
+
+// NewService constructs a Service over store.
+func NewService(store *Store) *Service { return &Service{store: store} }
+
+// Ping reports whether the payments schema is reachable.
+func (s *Service) Ping(ctx context.Context) error { return s.store.Ping(ctx) }

backend/internal/payments/store.go

@@ -0,0 +1,37 @@
+package payments
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+
+	"github.com/go-jet/jet/v2/postgres"
+
+	"scrabble/backend/internal/postgres/jet/payments/model"
+	"scrabble/backend/internal/postgres/jet/payments/table"
+)
+
+// Store is the Postgres-backed query surface for the payments schema. It is the
+// only place in the backend that issues SQL against payments.* (an
+// import-boundary test enforces it), so the domain stays extractable into its
+// own database.
+type Store struct {
+	db *sql.DB
+}
+
+// NewStore constructs a Store wrapping db.
+func NewStore(db *sql.DB) *Store { return &Store{db: db} }
+
+// Ping verifies the payments schema is reachable by reading the singleton config
+// row. It is the data-foundation health check; the wallet query surface arrives
+// with the currency mechanics.
+func (s *Store) Ping(ctx context.Context) error {
+	var row model.Config
+	if err := postgres.SELECT(table.Config.AllColumns).
+		FROM(table.Config).
+		LIMIT(1).
+		QueryContext(ctx, s.db, &row); err != nil {
+		return fmt.Errorf("payments: ping: %w", err)
+	}
+	return nil
+}