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.

gateway/internal/transcode/encode.go

@@ -184,6 +184,16 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	}
 	prefs := buildStringVector(b, p.VariantPreferences, fb.ProfileStartVariantPreferencesVector)
 	dictVersions := encodeDictVersions(b, p.DictVersions)
+	// The interstitial-ad config table (scalars only), built before Profile is opened.
+	var ads flatbuffers.UOffsetT
+	if p.Ads != nil {
+		fb.AdsInfoStart(b)
+		fb.AdsInfoAddCooldownGlobalS(b, int32(p.Ads.CooldownGlobalS))
+		fb.AdsInfoAddCooldownVsAiS(b, int32(p.Ads.CooldownVsAiS))
+		fb.AdsInfoAddCooldownHintS(b, int32(p.Ads.CooldownHintS))
+		fb.AdsInfoAddSuppressed(b, p.Ads.Suppressed)
+		ads = fb.AdsInfoEnd(b)
+	}
 	fb.ProfileStart(b)
 	fb.ProfileAddUserId(b, uid)
 	fb.ProfileAddDisplayName(b, name)
@@ -206,6 +216,9 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	if p.Banner != nil {
 		fb.ProfileAddBanner(b, banner)
 	}
+	if p.Ads != nil {
+		fb.ProfileAddAds(b, ads)
+	}
 	b.Finish(fb.ProfileEnd(b))
 	return b.FinishedBytes()
 }