feat(ads): server-driven ad-banner backend, wire & admin console (PR1)

Turn the gated-off mock banner into a real advertising subsystem (backend +
admin half; the UI rotation lands in PR2).

- internal/ads: campaigns (percent weight + validity window; a perpetual,
  undeletable default that fills the remainder up to 100%), 1..N bilingual
  messages (en+ru), global display timings; ActiveSet computes the
  window-filtered, default-remainder, GCD-reduced, language-resolved rotation
  feed. Smooth-weighted-round-robin math is unit-tested.
- migration 00006 (+ jetgen): ad_campaigns / ad_messages / ad_settings, seeded
  default campaign + house message + default timings.
- eligibility = !paid_account && hint_balance==0 && !no_banner role (new role;
  guests qualify). The resolved feed rides the profile.get response (no new RPC,
  works for guests, nothing distinct to filter); language by service_language.
- live update: a notify `banner` sub-kind (re-poll signal) published when an
  operator grants hints or grants/revokes no_banner, so the client shows/hides
  in place.
- admin console /_gm/banners (+ /_gm/banner-settings): campaign + message CRUD
  with reorder, default protection, clamped timings.
- wire: fbs BannerInfo/BannerCampaign on Profile; gateway transcode forwards it.
- docs: ARCHITECTURE §10, FUNCTIONAL (+ _ru), backend README, PRERELEASE tracker
  (incl. the deferred app.load aggregator note).

backend/internal/adminconsole/templates/layout.gohtml

@@ -20,6 +20,7 @@
     <a href="/_gm/messages"{{if eq .ActiveNav "messages"}} class="active"{{end}}>Messages</a>
     <a href="/_gm/throttled"{{if eq .ActiveNav "throttled"}} class="active"{{end}}>Throttled</a>
     <a href="/_gm/reasons"{{if eq .ActiveNav "reasons"}} class="active"{{end}}>Reasons</a>
+    <a href="/_gm/banners"{{if eq .ActiveNav "banners"}} class="active"{{end}}>Banners</a>
     <a href="/_gm/dictionary"{{if eq .ActiveNav "dictionary"}} class="active"{{end}}>Dictionary</a>
     <a href="/_gm/broadcast"{{if eq .ActiveNav "broadcast"}} class="active"{{end}}>Broadcast</a>
     <a href="/_gm/grafana/">Grafana ↗</a>