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.

backend/README.md

@@ -135,21 +135,23 @@ so `/internal/push-target` returns the recipient's `preferred_language` as the r
 language for out-of-app push; no per-bot routing remains. The console also manages the **advertising banner** (`/_gm/banners` +
 `/_gm/banner-settings`, `internal/ads`): operator campaigns with a percent weight, an optional
 window and bilingual messages, plus the global display timings. `GET /api/v1/user/profile` attaches
-the resolved, weighted campaign feed for an **eligible** viewer (`!paid_account && hint_balance == 0
-&& !no_banner` role, the message language picked by `preferred_language`); changing those inputs
-publishes a `notify` `banner` re-poll signal so the client shows/hides it in place. The shared wire
+the resolved, weighted campaign feed for an **eligible** viewer (no active **no-ads** benefit
+applicable in the current context and no **`no_banner`** role; the message language picked by
+`preferred_language`); changing those inputs
+publishes a `notify` `banner` re-poll signal so the client shows/hides it in place.
+The same gate drives the post-move interstitial config (`Profile.ads`, `adsFor`). The shared wire
 contracts live in the sibling [`../pkg`](../pkg) module.
 
 **Account linking & merge** (`/api/v1/user/link/*`). `internal/link`
 orchestrates it: an email confirm-code or a gateway-validated Telegram identity is
 attached to the current account, and when the identity already has its own account
-the two are merged in one transaction (`internal/accountmerge`) — stats and the hint
-wallet summed, `paid_account` ORed, identities/games/chat/complaints transferred,
+the two are merged in one transaction (`internal/accountmerge`) — stats summed,
+identities/games/chat/complaints transferred,
 friends/blocks de-duplicated, the secondary kept as a `merged_into` tombstone (so a
 shared finished game's foreign keys hold); a shared **active** game blocks the merge.
 The current account is primary, except a guest initiator whose linked identity has a
 durable owner — then the durable account wins and a fresh session is minted for it.
-The `accounts.paid_account`/`merged_into`/`merged_at` columns back this. This supersedes the
+The `accounts.merged_into`/`merged_at` columns back this. This supersedes the
 former `email.bind.*` edge surface (the `RequestCode`/`ConfirmCode` primitives stay).
 
 Rate-limit observability: the gateway posts its periodic rejection