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).

gateway/internal/transcode/encode.go

@@ -58,7 +58,8 @@ func encodeAck(ok bool) []byte {
 	return b.FinishedBytes()
 }
 
-// encodeProfile builds a Profile payload.
+// encodeProfile builds a Profile payload, including the advertising-banner block
+// when the backend marked the viewer eligible.
 func encodeProfile(p backendclient.ProfileResp) []byte {
 	b := flatbuffers.NewBuilder(192)
 	uid := b.CreateString(p.UserID)
@@ -67,6 +68,12 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	tz := b.CreateString(p.TimeZone)
 	awayStart := b.CreateString(p.AwayStart)
 	awayEnd := b.CreateString(p.AwayEnd)
+	// Build the banner table (and its children) before opening Profile: FlatBuffers
+	// forbids a nested table while another is under construction.
+	var banner flatbuffers.UOffsetT
+	if p.Banner != nil {
+		banner = encodeBanner(b, *p.Banner)
+	}
 	fb.ProfileStart(b)
 	fb.ProfileAddUserId(b, uid)
 	fb.ProfileAddDisplayName(b, name)
@@ -79,10 +86,51 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	fb.ProfileAddAwayStart(b, awayStart)
 	fb.ProfileAddAwayEnd(b, awayEnd)
 	fb.ProfileAddNotificationsInAppOnly(b, p.NotificationsInAppOnly)
+	if p.Banner != nil {
+		fb.ProfileAddBanner(b, banner)
+	}
 	b.Finish(fb.ProfileEnd(b))
 	return b.FinishedBytes()
 }
 
+// encodeBanner builds a BannerInfo table from the resolved banner block and
+// returns its offset. It is bottom-up: each campaign's messages vector and table
+// are built first, then the campaigns vector, then the BannerInfo table. The
+// caller must invoke it with no table under construction.
+func encodeBanner(b *flatbuffers.Builder, banner backendclient.BannerResp) flatbuffers.UOffsetT {
+	campOffsets := make([]flatbuffers.UOffsetT, len(banner.Campaigns))
+	for i, c := range banner.Campaigns {
+		msgOffsets := make([]flatbuffers.UOffsetT, len(c.Messages))
+		for j, m := range c.Messages {
+			msgOffsets[j] = b.CreateString(m)
+		}
+		fb.BannerCampaignStartMessagesVector(b, len(msgOffsets))
+		for j := len(msgOffsets) - 1; j >= 0; j-- {
+			b.PrependUOffsetT(msgOffsets[j])
+		}
+		msgs := b.EndVector(len(msgOffsets))
+		fb.BannerCampaignStart(b)
+		fb.BannerCampaignAddWeight(b, int32(c.Weight))
+		fb.BannerCampaignAddMessages(b, msgs)
+		campOffsets[i] = fb.BannerCampaignEnd(b)
+	}
+	fb.BannerInfoStartCampaignsVector(b, len(campOffsets))
+	for i := len(campOffsets) - 1; i >= 0; i-- {
+		b.PrependUOffsetT(campOffsets[i])
+	}
+	camps := b.EndVector(len(campOffsets))
+	t := banner.Timings
+	fb.BannerInfoStart(b)
+	fb.BannerInfoAddCampaigns(b, camps)
+	fb.BannerInfoAddHoldMs(b, int32(t.HoldMs))
+	fb.BannerInfoAddEdgePauseMs(b, int32(t.EdgePauseMs))
+	fb.BannerInfoAddScrollPxPerSec(b, int32(t.ScrollPxPerSec))
+	fb.BannerInfoAddFadeOutMs(b, int32(t.FadeOutMs))
+	fb.BannerInfoAddGapMs(b, int32(t.GapMs))
+	fb.BannerInfoAddFadeInMs(b, int32(t.FadeInMs))
+	return fb.BannerInfoEnd(b)
+}
+
 // encodeBlockStatus builds a BlockStatus payload.
 func encodeBlockStatus(s backendclient.BlockStatusResp) []byte {
 	b := flatbuffers.NewBuilder(128)