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).

pkg/fbs/scrabblefb/Profile.go

@@ -149,8 +149,21 @@ func (rcv *Profile) MutateNotificationsInAppOnly(n bool) bool {
 	return rcv._tab.MutateBoolSlot(24, n)
 }
 
+func (rcv *Profile) Banner(obj *BannerInfo) *BannerInfo {
+	o := flatbuffers.UOffsetT(rcv._tab.Offset(26))
+	if o != 0 {
+		x := rcv._tab.Indirect(o + rcv._tab.Pos)
+		if obj == nil {
+			obj = new(BannerInfo)
+		}
+		obj.Init(rcv._tab.Bytes, x)
+		return obj
+	}
+	return nil
+}
+
 func ProfileStart(builder *flatbuffers.Builder) {
-	builder.StartObject(11)
+	builder.StartObject(12)
 }
 func ProfileAddUserId(builder *flatbuffers.Builder, userId flatbuffers.UOffsetT) {
 	builder.PrependUOffsetTSlot(0, flatbuffers.UOffsetT(userId), 0)
@@ -185,6 +198,9 @@ func ProfileAddAwayEnd(builder *flatbuffers.Builder, awayEnd flatbuffers.UOffset
 func ProfileAddNotificationsInAppOnly(builder *flatbuffers.Builder, notificationsInAppOnly bool) {
 	builder.PrependBoolSlot(10, notificationsInAppOnly, true)
 }
+func ProfileAddBanner(builder *flatbuffers.Builder, banner flatbuffers.UOffsetT) {
+	builder.PrependUOffsetTSlot(11, flatbuffers.UOffsetT(banner), 0)
+}
 func ProfileEnd(builder *flatbuffers.Builder) flatbuffers.UOffsetT {
 	return builder.EndObject()
 }