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/payments/store_intake.go

@@ -385,6 +385,20 @@ type RewardOutcome struct {
 	AlreadyCredited bool
 }
 
+// interstitialCooldowns reads the post-move interstitial-ad cooldowns (seconds): the global
+// per-user cooldown, the longer vs_ai one, and the independent hint-triggered one. The client mirrors
+// them and self-gates (E6/D30).
+func (s *Store) interstitialCooldowns(ctx context.Context) (global, vsAi, hint int, err error) {
+	var cfg model.Config
+	if e := postgres.SELECT(table.Config.CooldownGlobalSeconds, table.Config.CooldownVsAiSeconds, table.Config.CooldownHintSeconds).
+		FROM(table.Config).
+		LIMIT(1).
+		QueryContext(ctx, s.db, &cfg); e != nil {
+		return 0, 0, 0, fmt.Errorf("payments: read interstitial cooldowns: %w", e)
+	}
+	return int(cfg.CooldownGlobalSeconds), int(cfg.CooldownVsAiSeconds), int(cfg.CooldownHintSeconds), nil
+}
+
 // rewardConfig reads the rewarded payout (chips per view) and the per-day and per-hour caps. The
 // caps are both anti-abuse (bounding a forger's free chips) and an economic conversion lever (free
 // rewarded chips are limited so a player who wants more buys) — tuned in the admin.