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/inttest/game_test.go

@@ -14,6 +14,8 @@ import (
 	"scrabble/backend/internal/account"
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
+	"scrabble/backend/internal/payments"
+	"scrabble/backend/internal/session"
 )
 
 // TestListForAccount checks the lobby "my games" query: it returns exactly the
@@ -401,8 +403,13 @@ func gameStatus(t *testing.T, svc *game.Service, id uuid.UUID) (status, endReaso
 // TestHintPolicy exercises the per-game allowance, the profile wallet and the
 // disabled switch.
 func TestHintPolicy(t *testing.T) {
-	ctx := context.Background()
+	// A trusted platform context so the online hint wallet (payments) is reachable; a provisioned
+	// account carries a telegram identity, so the telegram origin is the applicable one here.
+	ctx := session.WithPlatform(context.Background(),
+		session.Platform{Kind: session.PlatformKindTelegram, Subtype: session.SubtypeAndroid})
 	svc := newGameService()
+	pay := newPaymentsService()
+	svc.SetHintWallet(pay) // share the handle so a grant invalidates the wallet the game reads
 	seats := []uuid.UUID{provisionAccount(t), provisionAccount(t)}
 	seed := openingSeed(t)
 	g, err := svc.Create(ctx, game.CreateParams{
@@ -428,7 +435,11 @@ func TestHintPolicy(t *testing.T) {
 	if _, err := svc.Hint(ctx, g.ID, seats[0]); !errors.Is(err, game.ErrNoHintsLeft) {
 		t.Fatalf("second hint = %v, want ErrNoHintsLeft", err)
 	}
-	setHintBalance(t, seats[0], 2)
+	// Grant 2 hints on the telegram origin through the service so its read cache is invalidated
+	// (a raw insert would leave the cache warmed at zero by the allowance hint above).
+	if err := pay.Grant(ctx, seats[0], payments.SourceTelegram, 2, 0, false); err != nil {
+		t.Fatalf("grant hints: %v", err)
+	}
 	res, err := svc.Hint(ctx, g.ID, seats[0]) // spends the wallet
 	if err != nil {
 		t.Fatalf("wallet hint: %v", err)
@@ -603,14 +614,6 @@ func setAway(t *testing.T, id uuid.UUID, tz, start, end string) {
 	}
 }
 
-func setHintBalance(t *testing.T, id uuid.UUID, n int) {
-	t.Helper()
-	if _, err := testDB.ExecContext(context.Background(),
-		`UPDATE backend.accounts SET hint_balance = $1 WHERE account_id = $2`, n, id); err != nil {
-		t.Fatalf("set hint balance: %v", err)
-	}
-}
-
 func equalStrings(a, b []string) bool {
 	if len(a) != len(b) {
 		return false