feat(ads): VK post-move interstitial + retire deprecated hint_balance/paid_account

Interstitial video after a confirmed play or a hint, VK-only, offline
banner-only. The gate is client-mirrored: the backend puts the config
cooldowns (global 5m / vs_ai 30m / hint 1m) and a suppressed flag (the
no-ads / no_banner gate, same as the banner) on Profile.ads via adsFor;
the client self-gates on a per-kind last-shown time in localStorage, with
the VITE_ADS_STUB contour "ad fired" toast. Never fires after a pass,
exchange or resign. maybeShowInterstitial + vkShowInterstitial; the codec
and gateway transcode carry the ads block.

Also retires the deprecated accounts.hint_balance / paid_account domain
usage (expand-contract, code only — the columns stay for a later DROP so
image rollback stays DB-safe): drop the Account fields and their scan, the
dead account.SpendHint, account.GrantHints and the admin grant-hints
action; the in-game hint display now comes wholly from the payments hint
benefit. Banner eligibility and account merge no longer read the legacy
flags.

Tests: ads.test.ts (the client-mirrored gate), the codec ads round-trip,
the gateway ads transcode test, and a profile ads-config integration test
(cooldowns + suppressed under no-ads / no_banner). Docs: PAYMENTS §10 (+ru)
interstitial, the decision amends, backend README, the plan.

backend/internal/game/service.go

@@ -1272,10 +1272,6 @@ func (svc *Service) GameState(ctx context.Context, gameID, accountID uuid.UUID)
 	if !ok {
 		return StateView{}, ErrNotAPlayer
 	}
-	acc, err := svc.accounts.GetByID(ctx, accountID)
-	if err != nil {
-		return StateView{}, err
-	}
 
 	unlock := svc.locks.lock(gameID)
 	defer unlock()
@@ -1290,12 +1286,15 @@ func (svc *Service) GameState(ctx context.Context, gameID, accountID uuid.UUID)
 		}
 	}
 	return StateView{
-		Game:           pre,
-		Seat:           seat,
-		Rack:           g.Hand(seat),
-		BagLen:         g.BagLen(),
-		HintsRemaining: hintsRemaining(pre.HintsPerPlayer, pre.Seats[seat].HintsUsed, acc.HintBalance),
-		WalletBalance:  acc.HintBalance,
+		Game:   pre,
+		Seat:   seat,
+		Rack:   g.Hand(seat),
+		BagLen: g.BagLen(),
+		// The hint wallet moved to payments (svc.hintWallet); the deprecated accounts.hint_balance
+		// is no longer read, so the wire wallet is 0 and HintsRemaining is the per-seat allowance.
+		// The client adds the profile's payments hint balance on top (lib/hints.hintsLeft).
+		HintsRemaining: hintsRemaining(pre.HintsPerPlayer, pre.Seats[seat].HintsUsed, 0),
+		WalletBalance:  0,
 		// vs_ai idle-hint gate (seconds left; 0 for a human game / first move / not your turn).
 		HintUnlockLeftSeconds: hintUnlockLeftSeconds(pre, seat, svc.clock()),
 	}, nil