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.ts

@@ -4,6 +4,7 @@ export { AccountDeleteConfirm } from './scrabblefb/account-delete-confirm.js';
 export { AccountDeleteRequestResult } from './scrabblefb/account-delete-request-result.js';
 export { AccountRef } from './scrabblefb/account-ref.js';
 export { Ack } from './scrabblefb/ack.js';
+export { AdsInfo } from './scrabblefb/ads-info.js';
 export { AlphabetEntry } from './scrabblefb/alphabet-entry.js';
 export { BannerCampaign } from './scrabblefb/banner-campaign.js';
 export { BannerInfo } from './scrabblefb/banner-info.js';

ui/src/gen/fbs/scrabblefb/ads-info.ts

@@ -0,0 +1,76 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+import * as flatbuffers from 'flatbuffers';
+
+export class AdsInfo {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+  __init(i:number, bb:flatbuffers.ByteBuffer):AdsInfo {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+static getRootAsAdsInfo(bb:flatbuffers.ByteBuffer, obj?:AdsInfo):AdsInfo {
+  return (obj || new AdsInfo()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+static getSizePrefixedRootAsAdsInfo(bb:flatbuffers.ByteBuffer, obj?:AdsInfo):AdsInfo {
+  bb.setPosition(bb.position() + flatbuffers.SIZE_PREFIX_LENGTH);
+  return (obj || new AdsInfo()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+cooldownGlobalS():number {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
+cooldownVsAiS():number {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
+cooldownHintS():number {
+  const offset = this.bb!.__offset(this.bb_pos, 8);
+  return offset ? this.bb!.readInt32(this.bb_pos + offset) : 0;
+}
+
+suppressed():boolean {
+  const offset = this.bb!.__offset(this.bb_pos, 10);
+  return offset ? !!this.bb!.readInt8(this.bb_pos + offset) : false;
+}
+
+static startAdsInfo(builder:flatbuffers.Builder) {
+  builder.startObject(4);
+}
+
+static addCooldownGlobalS(builder:flatbuffers.Builder, cooldownGlobalS:number) {
+  builder.addFieldInt32(0, cooldownGlobalS, 0);
+}
+
+static addCooldownVsAiS(builder:flatbuffers.Builder, cooldownVsAiS:number) {
+  builder.addFieldInt32(1, cooldownVsAiS, 0);
+}
+
+static addCooldownHintS(builder:flatbuffers.Builder, cooldownHintS:number) {
+  builder.addFieldInt32(2, cooldownHintS, 0);
+}
+
+static addSuppressed(builder:flatbuffers.Builder, suppressed:boolean) {
+  builder.addFieldInt8(3, +suppressed, +false);
+}
+
+static endAdsInfo(builder:flatbuffers.Builder):flatbuffers.Offset {
+  const offset = builder.endObject();
+  return offset;
+}
+
+static createAdsInfo(builder:flatbuffers.Builder, cooldownGlobalS:number, cooldownVsAiS:number, cooldownHintS:number, suppressed:boolean):flatbuffers.Offset {
+  AdsInfo.startAdsInfo(builder);
+  AdsInfo.addCooldownGlobalS(builder, cooldownGlobalS);
+  AdsInfo.addCooldownVsAiS(builder, cooldownVsAiS);
+  AdsInfo.addCooldownHintS(builder, cooldownHintS);
+  AdsInfo.addSuppressed(builder, suppressed);
+  return AdsInfo.endAdsInfo(builder);
+}
+}

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;