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/server/server.go

@@ -28,6 +28,7 @@ import (
 	"scrabble/backend/internal/link"
 	"scrabble/backend/internal/lobby"
 	"scrabble/backend/internal/notify"
+	"scrabble/backend/internal/payments"
 	"scrabble/backend/internal/ratewatch"
 	"scrabble/backend/internal/render"
 	"scrabble/backend/internal/session"
@@ -93,6 +94,11 @@ type Deps struct {
 	// profile.get banner block, plus the banner admin console section. A nil Ads
 	// omits the banner block and disables the banner console.
 	Ads *ads.Service
+	// Payments is the in-game currency domain service (wallet, benefits, catalog).
+	// The data-foundation layer exposes only a reachability check; its user and
+	// console routes are registered when the wallet surface lands. A nil Payments
+	// omits them.
+	Payments *payments.Service
 	// Notifier publishes live-event intents — here the banner-eligibility re-poll
 	// signal the banner/hint/role console actions emit. A nil Notifier discards
 	// them (notify.Nop).
@@ -129,6 +135,7 @@ type Server struct {
 	ratewatch   *ratewatch.Watch
 	banview     *banview.View
 	ads         *ads.Service
+	payments    *payments.Service
 	notifier    notify.Publisher
 	console     *adminconsole.Renderer
 	exportKey   []byte
@@ -181,6 +188,7 @@ func New(addr string, deps Deps) *Server {
 		ratewatch:     deps.RateWatch,
 		banview:       deps.BanView,
 		ads:           deps.Ads,
+		payments:      deps.Payments,
 		notifier:      notifier,
 		renderer:      deps.Renderer,
 		http:          &http.Server{Addr: addr, Handler: engine},