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/notify/events.go

@@ -205,6 +205,13 @@ func notificationInvitation(userID uuid.UUID, sub string, inv InvitationSummary)
 	return Intent{UserID: userID, Kind: KindNotification, Payload: b.FinishedBytes(), EventID: eventID()}
 }
 
+// BannerChanged tells userID that its advertising-banner eligibility may have
+// changed, so the client re-fetches profile.get to show or hide the banner. It
+// carries no payload (the banner set rides the profile response).
+func BannerChanged(userID uuid.UUID) Intent {
+	return Notification(userID, NotifyBanner)
+}
+
 // eventID returns a best-effort correlation id for one emitted event.
 func eventID() string {
 	if id, err := uuid.NewV7(); err == nil {