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/vk.ts

@@ -225,6 +225,20 @@ export async function vkShowRewarded(): Promise<boolean> {
   }
 }
 
+/**
+ * vkShowInterstitial shows a between-screens interstitial ad (VKWebAppShowNativeAds, interstitial
+ * format) and reports whether it was shown. It carries no reward — it is the post-move fullscreen ad.
+ * A rejected or unavailable call reports false.
+ */
+export async function vkShowInterstitial(): Promise<boolean> {
+  try {
+    const data = await (await bridge()).send('VKWebAppShowNativeAds', { ad_format: 'interstitial' });
+    return !!(data as { result?: boolean }).result;
+  } catch {
+    return false;
+  }
+}
+
 /**
  * vkDownloadFile downloads url as filename through the VK client (VKWebAppDownloadFile) —
  * the mobile in-app file delivery, where the webview ignores <a download>. Resolves false