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/game/Game.svelte

@@ -15,6 +15,7 @@
   import { app, handleError, showToast, markChatRead, seedChatUnread } from '../lib/app.svelte';
   import { connection } from '../lib/connection.svelte';
   import { offlineMode } from '../lib/offline.svelte';
+  import { maybeShowInterstitial } from '../lib/ads';
   import { GatewayError } from '../lib/client';
   import { t, type MessageKey } from '../lib/i18n/index.svelte';
   import type { EvalResult, MoveRecord, MoveResult, StateView, Tile } from '../lib/model';
@@ -946,6 +947,12 @@
       if (view?.game.hotseat) await advanceHotseat();
       haptic('success');
       zoomed = false;
+      // A confirmed move may trigger a post-move interstitial (VK, frequency-gated, client-mirrored).
+      // Only submitPlay earns one — never a pass / exchange / resign. Fire-and-forget.
+      void maybeShowInterstitial(app.profile?.ads, 'move', {
+        vsAi: !!view?.game.vsAi,
+        online: connection.online && !offlineMode.active,
+      });
     } catch (e) {
       handleError(e);
     } finally {
@@ -1007,6 +1014,11 @@
     }
     try {
       const h = await source.hint(id);
+      // Applying a hint triggers its own post-move interstitial, independent of the move cooldown.
+      void maybeShowInterstitial(app.profile?.ads, 'hint', {
+        vsAi: !!view?.game.vsAi,
+        online: connection.online && !offlineMode.active,
+      });
       if (h.move.tiles.length && view) {
         placement = placementFromHint(h.move.tiles, view.rack);
         // Scroll the (zoomed) board to the hint's placement rather than the top-left: