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.

PLAN.md

@@ -35,7 +35,7 @@ status — without re-deriving decisions.
 | E3 | Wallet UI | 1 | DONE |
 | E4 | Durability (PITR) | 2 | DONE |
 | E5 | Payment intake | 2 | DONE |
-| E6 | Ads | 2 | WIP |
+| E6 | Ads | 2 | DONE |
 | E7 | Admin & reports | 2 | TODO |
 | E8 | Guest limits | — | TODO |
 | E9 | Tournament fee | future | TODO |
@@ -618,7 +618,7 @@ force-recreate when the Caddyfile changes.
 
 ## E6 — Ads
 
-**Status:** WIP · **Release 2** · depends on: E2 (chips), E5 (rewarded credits via intake) ·
+**Status:** DONE · **Release 2** · depends on: E2 (chips), E5 (rewarded credits via intake) ·
 mechanics: PAYMENTS §10.
 
 **Delivery & baked decisions.** Shipped as a linear PR stack (owner's choice): **rewarded first**,
@@ -640,8 +640,19 @@ VK returns **only `{result:true}`** (no token/signature) — client-attested is
 possible; the diagnostic is removed. The slice also **corrects the VK-iOS freeze to purchase-only**
 (rewarded on VK-iOS earns chips, which the old blanket "spend freeze" then blocked from spending —
 Apple forbids only *buying* in-app values, not spending or earning them; `vkFrozen()` now gates only
-`CreateOrder`, not `spendableSources`, so VK-wallet chips spend on VK-iOS). The legacy `paid_account`
-/ `hint_balance` drop (D31) and the **interstitial** are the next slice.
+`CreateOrder`, not `spendableSources`, so VK-wallet chips spend on VK-iOS). Delivered on
+`feature/ads-interstitial` (the **interstitial + D31** slice): the post-move fullscreen interstitial
+as a **client-mirrored** gate — the backend `adsFor` puts the config cooldowns + a `suppressed` flag
+(the no-ads / `no_banner` gate, same as the banner) on the profile (`Profile.ads`), and
+`ui/src/lib/ads.ts` `maybeShowInterstitial` self-gates on the last-shown time per kind in
+`localStorage`, showing a VK interstitial (`vkShowInterstitial`) after a **confirmed play or a hint
+only** (never a pass / exchange / resign), VK-only, offline banner-only, with the same `VITE_ADS_STUB`
+toast on the contour. The slice also lands **D31 step 1 (contract-code)**: the domain no longer reads
+or writes the deprecated `accounts.hint_balance` / `paid_account` columns — the `Account` fields, the
+dead `account.SpendHint`, `account.GrantHints` and the admin **grant-hints** action are removed, and
+the in-game hint display now comes wholly from the payments benefit (`HintsAvailable`). The **columns
+stay** (no migration → image rollback is DB-safe); a later contract-PR does the `DROP` once E6 is
+stable on prod.
 
 **Goal.** VK video ads: the post-move interstitial (frequency-gated) and the rewarded video
 (credits chips via server verify), plus extending the existing banner suppression to
@@ -657,8 +668,9 @@ per-origin.
 - **Interstitial** (post-move fullscreen), configurable server values (from `payments`
   config): global per-user cooldown across all games (default 5 min); `vs_ai` 30 min; a hint
   application triggers a post-move interstitial independently with its own 1-min cooldown;
-  offline banner-only; respect VK's own frequency caps. Cooldown state tracked server-side
-  (per user) or client-mirrored from a server value — pick and document at implementation.
+  offline banner-only; respect VK's own frequency caps. Cooldown state is **client-mirrored**
+  (the chosen option): the server sends the cooldowns + `suppressed` on the profile and the
+  client self-gates on a per-kind last-shown time in `localStorage` — no per-move round-trip.
 - **Banner suppression:** extend `ads.Eligible` (`backend/internal/ads/ads.go` :107) to gate
   on the **origin benefit applicable in the current context** (E2 interface) instead of the
   single legacy flag. No-ads suppresses banner + interstitial; rewarded never suppressed.