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.

docs/PAYMENTS.md

@@ -280,12 +280,19 @@ not suppressed by no-ads (D9). A future network that offers a server verify slot
 abstraction. On the test contour a build flag (`VITE_ADS_STUB`) swaps a toast for the real ad; prod
 always shows real ads (a failed real ad must not credit).
 
-**Interstitial** (post-move fullscreen), configurable server-side values:
+**Interstitial** (fullscreen after a confirmed play), **VK-only**, configurable server-side
+values. The gate is **client-mirrored**: the profile carries the cooldowns and a `suppressed`
+flag (the same no-ads / `no_banner` gate as the banner, resolved server-side in `adsFor`), and
+the client self-gates on the last-shown time **per kind** in `localStorage` — no per-move
+server round-trip. The contour `VITE_ADS_STUB` swaps the same "ad fired" toast for the real ad.
+Values:
 
 - Global cooldown **per user, across all games**, default **5 min**.
 - **`vs_ai` — 30 min** (aligned with the hint cooldown, so it does not scare casual players).
-- Applying a **hint** triggers a post-move interstitial **independently** of the main
-  cooldown, with its own **1-min** cooldown.
+- Applying a **hint** triggers an interstitial **independently** of the main cooldown, with
+  its own **1-min** cooldown.
+- Fires **only after a confirmed play or a hint** — never after a pass, exchange or resign
+  (an ad for a non-scoring action would only annoy, so those are never "rewarded" with one).
 - Offline — banner only.
 - Respect VK's own frequency caps.