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/model.ts

@@ -226,6 +226,8 @@ export interface Profile {
   variantPreferences: Variant[];
   /** The advertising-banner block, present only for a viewer eligible to see it. */
   banner?: Banner;
+  /** The post-move interstitial-ad config for the client-mirrored gate (cooldowns + suppressed). */
+  ads?: AdsConfig;
   /** The account's confirmed email address ("" when none). */
   email: string;
   /** Whether a Telegram / VK identity is attached — drives the Add / Unlink controls. */
@@ -237,6 +239,15 @@ export interface Profile {
   dictVersions: Partial<Record<Variant, string>>;
 }
 
+/** The post-move interstitial-ad config for the client-mirrored gate: the cooldowns (seconds) and
+ *  whether ads are suppressed in the current context (a no-ads benefit here, or the no_banner role). */
+export interface AdsConfig {
+  cooldownGlobalS: number;
+  cooldownVsAiS: number;
+  cooldownHintS: number;
+  suppressed: boolean;
+}
+
 /** Banner is the advertising-banner block of an eligible viewer's profile. */
 export interface Banner {
   campaigns: BannerCampaign[];