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/ads.ts

@@ -4,7 +4,9 @@
 // flow is testable without waiting for a real ad — production never sets the flag, so a real ad that
 // fails to load there must not credit (no stub fallback in prod).
 import { executionContext } from './wallet';
-import { vkRewardedReady, vkShowRewarded } from './vk';
+import { vkRewardedReady, vkShowRewarded, vkShowInterstitial } from './vk';
+import { showToast } from './app.svelte';
+import type { AdsConfig } from './model';
 
 /**
  * adsStubEnabled reports the contour test stub (VITE_ADS_STUB=1). Production never sets it, so it
@@ -46,3 +48,55 @@ export async function showRewarded(): Promise<RewardedResult> {
   }
   return { watched: await vkShowRewarded(), stub: false };
 }
+
+// The post-move interstitial gate is client-mirrored: the server sends the cooldowns + suppressed in
+// the profile (Profile.ads); the client tracks the last-shown time per kind in localStorage and
+// self-gates. Last-shown is per-device (cleared with storage) — acceptable for a frequency gate.
+const INTERSTITIAL_LAST_KEY = 'ads.interstitial.last';
+
+// interstitialLast reads the last-shown epoch millis per kind (0 when absent/corrupt).
+function interstitialLast(): { move: number; hint: number } {
+  try {
+    const raw = localStorage.getItem(INTERSTITIAL_LAST_KEY);
+    if (raw) {
+      const j = JSON.parse(raw) as Partial<{ move: number; hint: number }>;
+      return { move: j.move ?? 0, hint: j.hint ?? 0 };
+    }
+  } catch {
+    /* a corrupt or unavailable store simply resets the gate */
+  }
+  return { move: 0, hint: 0 };
+}
+
+function recordInterstitial(kind: 'move' | 'hint', now: number): void {
+  try {
+    const l = interstitialLast();
+    l[kind] = now;
+    localStorage.setItem(INTERSTITIAL_LAST_KEY, JSON.stringify(l));
+  } catch {
+    /* ignore a write failure — the worst case is one extra ad next time */
+  }
+}
+
+/**
+ * maybeShowInterstitial shows a post-move (`kind='move'`, global / vs_ai cooldown) or post-hint
+ * (`kind='hint'`, its own short cooldown) interstitial when the client-mirrored gate allows: not
+ * suppressed (no-ads), online, inside VK (the stub bypasses VK), and the mirrored cooldown elapsed.
+ * The stub (contour test) shows an "ad fired" toast. Returns whether an ad was shown. Fire-and-forget
+ * — the caller does not await a result to proceed.
+ */
+export async function maybeShowInterstitial(
+  ads: AdsConfig | undefined,
+  kind: 'move' | 'hint',
+  opts: { vsAi: boolean; online: boolean },
+): Promise<boolean> {
+  if (!ads || ads.suppressed || !opts.online) return false;
+  const stub = adsStubEnabled();
+  if (!stub && executionContext() !== 'vk') return false;
+  const cooldownS = kind === 'hint' ? ads.cooldownHintS : opts.vsAi ? ads.cooldownVsAiS : ads.cooldownGlobalS;
+  const now = Date.now();
+  if (now - interstitialLast()[kind] < cooldownS * 1000) return false;
+  const shown = stub ? (showToast('ad fired'), true) : await vkShowInterstitial();
+  if (shown) recordInterstitial(kind, now);
+  return shown;
+}