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.

ui/src/lib/mock/data.ts

@@ -45,6 +45,8 @@ export const PROFILE: Profile = {
   // Every variant's current dictionary version, as the backend advertises it on the profile —
   // the offline preloader targets `variant@version`.
   dictVersions: { erudit_ru: 'v1.3.0', scrabble_ru: 'v1.3.0', scrabble_en: 'v1.3.0' },
+  // Post-move interstitial config (short cooldowns so the mock stub is exercisable); not suppressed.
+  ads: { cooldownGlobalS: 300, cooldownVsAiS: 1800, cooldownHintS: 60, suppressed: false },
 };
 
 // Seed social/account data for the mock (pnpm start + Playwright). The mock profile