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/cmd/backend/main.go

@@ -19,6 +19,7 @@ import (
 
 	"scrabble/backend/internal/account"
 	"scrabble/backend/internal/accountmerge"
+	"scrabble/backend/internal/ads"
 	"scrabble/backend/internal/config"
 	"scrabble/backend/internal/connector"
 	"scrabble/backend/internal/engine"
@@ -200,6 +201,10 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		zap.Int("flag_threshold", cfg.RateWatch.FlagThreshold),
 		zap.Duration("flag_window", cfg.RateWatch.FlagWindow))
 
+	// Advertising-banner domain: campaign rotation feeding the profile.get banner
+	// block and the banner admin console section.
+	adsSvc := ads.NewService(ads.NewStore(db))
+
 	srv := server.New(cfg.HTTPAddr, server.Deps{
 		Logger:        logger,
 		DB:            db,
@@ -218,6 +223,8 @@ func run(ctx context.Context, cfg config.Config, logger *zap.Logger) error {
 		DictDir:       cfg.Game.DictDir,
 		Connector:     conn,
 		RateWatch:     rateWatch,
+		Ads:           adsSvc,
+		Notifier:      hub,
 	})
 	pushSrv := pushgrpc.NewServer(cfg.GRPCAddr, hub, logger)