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/templates/pages/user_detail.gohtml

@@ -10,8 +10,6 @@
 <li><b>Timezone</b> {{.TimeZone}}</li>
 <li><b>Guest</b> {{if .Guest}}yes{{else}}no{{end}}</li>
 <li><b>Push</b> {{if .NotificationsInAppOnly}}in-app only{{else}}out-of-app{{end}}</li>
-<li><b>Paid</b> {{if .PaidAccount}}yes{{else}}no{{end}}</li>
-<li><b>Hint wallet</b> {{.HintBalance}}</li>
 {{if .MergedInto}}<li><b>Merged into</b> {{.MergedInto}}</li>{{end}}
 {{if .FlaggedHighRateAt}}<li><b>High-rate flag</b> <span class="warn">{{.FlaggedHighRateAt}}</span></li>{{end}}
 <li><b>Created</b> {{.CreatedAt}}</li>
@@ -21,10 +19,6 @@
 <button type="submit">Clear high-rate flag</button>
 </form>
 {{end}}
-<form class="form" method="post" action="/_gm/users/{{.ID}}/grant-hints">
-<label>Add hints <input type="number" name="amount" min="1" max="{{.HintGrantMax}}" value="1"></label>
-<button type="submit">Grant</button>
-</form>
 </section>
 <section class="panel"><h2>Statistics</h2>
 {{if .HasStats}}