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/codec.test.ts

@@ -814,6 +814,42 @@ describe('codec', () => {
     expect(decodeProfile(b.asUint8Array()).banner).toBeUndefined();
   });
 
+  it('decodes the profile interstitial-ad config', () => {
+    const b = new Builder(64);
+    const uid = b.createString('u-1');
+    const ads = fb.AdsInfo.createAdsInfo(b, 300, 1800, 60, false);
+    fb.Profile.startProfile(b);
+    fb.Profile.addUserId(b, uid);
+    fb.Profile.addAds(b, ads);
+    b.finish(fb.Profile.endProfile(b));
+    expect(decodeProfile(b.asUint8Array()).ads).toEqual({
+      cooldownGlobalS: 300,
+      cooldownVsAiS: 1800,
+      cooldownHintS: 60,
+      suppressed: false,
+    });
+  });
+
+  it('carries the suppressed flag through the ad config', () => {
+    const b = new Builder(64);
+    const uid = b.createString('u-1');
+    const ads = fb.AdsInfo.createAdsInfo(b, 300, 1800, 60, true);
+    fb.Profile.startProfile(b);
+    fb.Profile.addUserId(b, uid);
+    fb.Profile.addAds(b, ads);
+    b.finish(fb.Profile.endProfile(b));
+    expect(decodeProfile(b.asUint8Array()).ads?.suppressed).toBe(true);
+  });
+
+  it('leaves the profile ads undefined when absent', () => {
+    const b = new Builder(64);
+    const uid = b.createString('u-1');
+    fb.Profile.startProfile(b);
+    fb.Profile.addUserId(b, uid);
+    b.finish(fb.Profile.endProfile(b));
+    expect(decodeProfile(b.asUint8Array()).ads).toBeUndefined();
+  });
+
   it('decodes the profile variant preferences', () => {
     const b = new Builder(64);
     const uid = b.createString('u-1');