feat(payments): chip wallet, store-compliance gate and benefit application

Stand up the internal chip/benefit mechanic behind the narrow payments interface:
context-aware balances and benefits, an atomic chip spend, admin grants as
zero-price value sales, the one-directional store-compliance gate (VK/TG same-
origin only, web draws direct→vk→tg, VK-iOS frozen, untrusted fail-closed), and
per-origin hint and no-ads application with term stacking. Reads are served from
an in-process, account-keyed write-through cache (mirroring the suspension gate),
so hot paths issue no query to the payments schema.

Flip the online-game hint wallet and the ad-banner suppression from the deprecated
accounts.hint_balance / paid_account columns to the payments benefit (a hint
balance no longer suppresses the banner — only a no-ads benefit does), and fold
chip segments and benefits by origin on account merge, inside the merge tx. Add
the GET/POST /api/v1/user/wallet edge chain (REST → Connect → FlatBuffers) plus
its codec unit test; no wallet UI yet.

Bring the frozen owner decisions log into the repo at
docs/PAYMENTS_DECISIONS_ru.md (it was untracked under .vscode) and reference it
from PLAN.md; record the read-cache design and the present-sources interface in
PLAN.md and docs/PAYMENTS.md (+ RU mirror).

backend/internal/accountmerge/merge.go

@@ -51,8 +51,24 @@ var ErrSameAccount = errors.New("accountmerge: primary and secondary are the sam
 type Merger struct {
 	db  *sql.DB
 	now func() time.Time
+	// payments, when set, merges the two accounts' chip segments and benefits by origin inside
+	// the merge transaction (SetPayments). Nil leaves payments untouched (tests that do not
+	// exercise the wallet).
+	payments PaymentsMerger
 }
 
+// PaymentsMerger is the payments surface the account merge enlists: fold the secondary's
+// segments and benefits into the primary within the merge transaction, then invalidate the
+// affected read caches after the commit. *payments.Service satisfies it.
+type PaymentsMerger interface {
+	MergeTx(ctx context.Context, tx *sql.Tx, primary, secondary uuid.UUID) error
+	Invalidate(ids ...uuid.UUID)
+}
+
+// SetPayments installs the payments merge hook. It must be called during startup wiring; the
+// default (nil) merges no wallet state.
+func (m *Merger) SetPayments(p PaymentsMerger) { m.payments = p }
+
 // NewMerger constructs a Merger over db.
 func NewMerger(db *sql.DB) *Merger {
 	return &Merger{db: db, now: func() time.Time { return time.Now().UTC() }}
@@ -67,7 +83,7 @@ func (m *Merger) Merge(ctx context.Context, primary, secondary uuid.UUID) error
 		return ErrSameAccount
 	}
 	now := m.now()
-	return withTx(ctx, m.db, func(tx *sql.Tx) error {
+	if err := withTx(ctx, m.db, func(tx *sql.Tx) error {
 		if err := guardActiveSharedGame(ctx, tx, primary, secondary); err != nil {
 			return err
 		}
@@ -107,8 +123,21 @@ func (m *Merger) Merge(ctx context.Context, primary, secondary uuid.UUID) error
 		if err := deleteEphemerals(ctx, tx, secondary); err != nil {
 			return err
 		}
+		if m.payments != nil {
+			if err := m.payments.MergeTx(ctx, tx, primary, secondary); err != nil {
+				return fmt.Errorf("accountmerge: payments: %w", err)
+			}
+		}
 		return tombstone(ctx, tx, primary, secondary, now)
-	})
+	}); err != nil {
+		return err
+	}
+	// The payments read cache is invalidated only after the merge commits, so a read racing the
+	// transaction cannot re-cache pre-merge state (both accounts' rows are moved or dropped).
+	if m.payments != nil {
+		m.payments.Invalidate(primary, secondary)
+	}
+	return nil
 }
 
 // guardActiveSharedGame returns ErrActiveGameConflict when primary and secondary
@@ -248,25 +277,16 @@ func mergeBestMoves(ctx context.Context, tx *sql.Tx, primary, secondary uuid.UUI
 	return nil
 }
 
-// mergeAccountFields adds secondary's hint wallet to primary and ORs the paid flag;
-// all other profile fields stay the primary's.
-func mergeAccountFields(ctx context.Context, tx *sql.Tx, primary, secondary uuid.UUID, now time.Time) error {
-	var sec model.Accounts
-	if err := postgres.SELECT(table.Accounts.AllColumns).
-		FROM(table.Accounts).
-		WHERE(table.Accounts.AccountID.EQ(postgres.UUID(secondary))).
-		QueryContext(ctx, tx, &sec); err != nil {
-		return fmt.Errorf("accountmerge: load secondary account: %w", err)
-	}
-	upd := table.Accounts.UPDATE(
-		table.Accounts.HintBalance, table.Accounts.PaidAccount, table.Accounts.UpdatedAt,
-	).SET(
-		table.Accounts.HintBalance.ADD(postgres.Int(int64(sec.HintBalance))),
-		table.Accounts.PaidAccount.OR(postgres.Bool(sec.PaidAccount)),
-		postgres.TimestampzT(now),
-	).WHERE(table.Accounts.AccountID.EQ(postgres.UUID(primary)))
+// mergeAccountFields bumps the primary account's updated_at to reflect the merge. The former
+// hint-wallet and paid-flag merge moved to the payments domain, where segments and benefits
+// merge by origin (see the payments MergeTx step and docs/PAYMENTS.md §6); the legacy
+// accounts.hint_balance / paid_account columns are deprecated and no longer read or written.
+func mergeAccountFields(ctx context.Context, tx *sql.Tx, primary, _ uuid.UUID, now time.Time) error {
+	upd := table.Accounts.UPDATE(table.Accounts.UpdatedAt).
+		SET(postgres.TimestampzT(now)).
+		WHERE(table.Accounts.AccountID.EQ(postgres.UUID(primary)))
 	if _, err := upd.ExecContext(ctx, tx); err != nil {
-		return fmt.Errorf("accountmerge: update primary account: %w", err)
+		return fmt.Errorf("accountmerge: touch primary account: %w", err)
 	}
 	return nil
 }