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/cmd/backend/main.go

@@ -195,7 +195,8 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	emails.SetSendLimiter(account.NewSendLimiter(time.Minute, 5))
 	// Account linking & merge: the orchestrator over the account, merge and
 	// session layers. Wired to the /api/v1/user/link REST surface below.
-	links := link.NewService(emails, accounts, accountmerge.NewMerger(db), sessions)
+	merger := accountmerge.NewMerger(db)
+	links := link.NewService(emails, accounts, merger, sessions)
 	socialSvc := social.NewService(social.NewStore(db), accounts, games)
 	socialSvc.SetNotifier(hub)
 	socialSvc.SetMetrics(tel.MeterProvider().Meter("scrabble/backend/social"))
@@ -270,6 +271,12 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 	}
 	logger.Info("payments domain ready")
 
+	// Wire the payments surface into the domains that consume it: the online-game hint wallet
+	// and the account-merge wallet fold. Done after the reachability check so a broken payments
+	// schema fails boot before anything depends on it.
+	games.SetHintWallet(paymentsSvc)
+	merger.SetPayments(paymentsSvc)
+
 	// The image-render sidecar client for the PNG export artifact; nil (PNG
 	// download answers 404) when BACKEND_RENDERER_URL is unset.
 	var renderer *render.Client