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.

pkg/fbs/scrabble.fbs

@@ -264,6 +264,18 @@ table Profile {
   // preload the right dictionary and pin a new local game without a separate request (added
   // trailing — backward-compatible).
   dict_versions:[DictVersion];
+  // ads carries the post-move interstitial config (cooldowns + suppressed) for the client-mirrored
+  // gate (added trailing — backward-compatible).
+  ads:AdsInfo;
+}
+
+// AdsInfo is the post-move interstitial config: the client-mirrored cooldowns (seconds) and whether
+// ads are suppressed in the caller's context (a no-ads benefit here, or the no_banner role).
+table AdsInfo {
+  cooldown_global_s:int;
+  cooldown_vs_ai_s:int;
+  cooldown_hint_s:int;
+  suppressed:bool;
 }
 
 // BlockStatus reports the caller's current manual block. The UI fetches it after any operation