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/internal/adminconsole/views.go

@@ -149,7 +149,6 @@ type UserDetailView struct {
 	TimeZone               string
 	Guest                  bool
 	NotificationsInAppOnly bool
-	PaidAccount            bool
 	// MergedInto is the primary account id when this account has been retired by a
 	// merge, or empty for a live account.
 	MergedInto string
@@ -165,14 +164,10 @@ type UserDetailView struct {
 	// FlaggedHighRateAt is the pre-formatted soft high-rate marker timestamp,
 	// empty for an unflagged account; the card shows it with the Clear action.
 	FlaggedHighRateAt string
-	HintBalance       int
-	// HintGrantMax is the per-grant cap the operator's "add hints" form enforces (it mirrors the
-	// server's maxHintGrant), passed through so the policy value lives in one place.
-	HintGrantMax int
-	CreatedAt    string
-	HasStats     bool
-	Stats        StatsRow
-	Identities   []IdentityRow
+	CreatedAt         string
+	HasStats          bool
+	Stats             StatsRow
+	Identities        []IdentityRow
 	// HasEmail gates the "Erase email" action; set when the account carries an email identity.
 	HasEmail bool
 	Games    []GameRow