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/gen/fbs/scrabblefb/profile.ts

@@ -2,6 +2,7 @@
 
 import * as flatbuffers from 'flatbuffers';
 
+import { AdsInfo } from '../scrabblefb/ads-info.js';
 import { BannerInfo } from '../scrabblefb/banner-info.js';
 import { DictVersion } from '../scrabblefb/dict-version.js';
 
@@ -135,8 +136,13 @@ dictVersionsLength():number {
   return offset ? this.bb!.__vector_len(this.bb_pos + offset) : 0;
 }
 
+ads(obj?:AdsInfo):AdsInfo|null {
+  const offset = this.bb!.__offset(this.bb_pos, 38);
+  return offset ? (obj || new AdsInfo()).__init(this.bb!.__indirect(this.bb_pos + offset), this.bb!) : null;
+}
+
 static startProfile(builder:flatbuffers.Builder) {
-  builder.startObject(17);
+  builder.startObject(18);
 }
 
 static addUserId(builder:flatbuffers.Builder, userIdOffset:flatbuffers.Offset) {
@@ -231,6 +237,10 @@ static startDictVersionsVector(builder:flatbuffers.Builder, numElems:number) {
   builder.startVector(4, numElems, 4);
 }
 
+static addAds(builder:flatbuffers.Builder, adsOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(17, adsOffset, 0);
+}
+
 static endProfile(builder:flatbuffers.Builder):flatbuffers.Offset {
   const offset = builder.endObject();
   return offset;