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/backendclient/api.go

@@ -34,6 +34,33 @@ type ProfileResp struct {
 	BlockFriendRequests    bool   `json:"block_friend_requests"`
 	IsGuest                bool   `json:"is_guest"`
 	NotificationsInAppOnly bool   `json:"notifications_in_app_only"`
+	// Banner is the advertising-banner block, present only for a viewer eligible to
+	// see the banner. The gateway forwards it verbatim into the Profile payload.
+	Banner *BannerResp `json:"banner,omitempty"`
+}
+
+// BannerResp is the advertising-banner block of an eligible viewer's profile: the
+// campaigns to rotate and the global display timings the client rotator reads.
+type BannerResp struct {
+	Campaigns []BannerCampaignResp `json:"campaigns"`
+	Timings   BannerTimingsResp    `json:"timings"`
+}
+
+// BannerCampaignResp is one campaign in the rotation feed: a GCD-reduced show
+// weight and its messages, in display order, already resolved to one language.
+type BannerCampaignResp struct {
+	Weight   int      `json:"weight"`
+	Messages []string `json:"messages"`
+}
+
+// BannerTimingsResp mirrors the backend's global display timings.
+type BannerTimingsResp struct {
+	HoldMs         int `json:"hold_ms"`
+	EdgePauseMs    int `json:"edge_pause_ms"`
+	ScrollPxPerSec int `json:"scroll_px_per_sec"`
+	FadeOutMs      int `json:"fade_out_ms"`
+	GapMs          int `json:"gap_ms"`
+	FadeInMs       int `json:"fade_in_ms"`
 }
 
 // LinkResultResp is the result of an account link/merge step. Status is