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/transcode_banner_test.go

@@ -0,0 +1,81 @@
+package transcode_test
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	"scrabble/gateway/internal/transcode"
+	fb "scrabble/pkg/fbs/scrabblefb"
+)
+
+// TestProfileGetEncodesBanner verifies the gateway forwards the backend's banner
+// block verbatim into the Profile payload: the campaigns (weight + resolved
+// messages, in order) and the global display timings.
+func TestProfileGetEncodesBanner(t *testing.T) {
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		if r.Method != http.MethodGet || r.URL.Path != "/api/v1/user/profile" {
+			t.Errorf("unexpected %s %q", r.Method, r.URL.Path)
+		}
+		_, _ = w.Write([]byte(`{"user_id":"u-1","display_name":"Kaya","preferred_language":"en",` +
+			`"banner":{"campaigns":[` +
+			`{"weight":3,"messages":["promo-en"]},` +
+			`{"weight":7,"messages":["house-a","house-b"]}],` +
+			`"timings":{"hold_ms":60000,"edge_pause_ms":5000,"scroll_px_per_sec":40,` +
+			`"fade_out_ms":1000,"gap_ms":250,"fade_in_ms":1000}}}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, ok := reg.Lookup(transcode.MsgProfileGet)
+	if !ok {
+		t.Fatal("profile.get not registered")
+	}
+	payload, err := op.Handler(context.Background(), transcode.Request{UserID: "u-1"})
+	if err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+
+	p := fb.GetRootAsProfile(payload, 0)
+	banner := p.Banner(nil)
+	if banner == nil {
+		t.Fatal("profile carries no banner block")
+	}
+	if got := banner.CampaignsLength(); got != 2 {
+		t.Fatalf("campaigns = %d, want 2", got)
+	}
+	var c fb.BannerCampaign
+	banner.Campaigns(&c, 0)
+	if c.Weight() != 3 || c.MessagesLength() != 1 || string(c.Messages(0)) != "promo-en" {
+		t.Errorf("campaign 0 = weight %d, %d msgs, msg0=%q", c.Weight(), c.MessagesLength(), c.Messages(0))
+	}
+	banner.Campaigns(&c, 1)
+	if c.Weight() != 7 || c.MessagesLength() != 2 || string(c.Messages(1)) != "house-b" {
+		t.Errorf("campaign 1 = weight %d, %d msgs, msg1=%q", c.Weight(), c.MessagesLength(), c.Messages(1))
+	}
+	if banner.HoldMs() != 60000 || banner.EdgePauseMs() != 5000 || banner.ScrollPxPerSec() != 40 {
+		t.Errorf("timings = hold %d edge %d scroll %d", banner.HoldMs(), banner.EdgePauseMs(), banner.ScrollPxPerSec())
+	}
+	if banner.FadeOutMs() != 1000 || banner.GapMs() != 250 || banner.FadeInMs() != 1000 {
+		t.Errorf("fade timings = out %d gap %d in %d", banner.FadeOutMs(), banner.GapMs(), banner.FadeInMs())
+	}
+}
+
+// TestProfileGetNoBanner verifies an ineligible viewer's profile carries no
+// banner block (the backend omits it).
+func TestProfileGetNoBanner(t *testing.T) {
+	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte(`{"user_id":"u-1","display_name":"Kaya","preferred_language":"en"}`))
+	})
+	defer cleanup()
+
+	reg := transcode.NewRegistry(backend, nil)
+	op, _ := reg.Lookup(transcode.MsgProfileGet)
+	payload, err := op.Handler(context.Background(), transcode.Request{UserID: "u-1"})
+	if err != nil {
+		t.Fatalf("handler: %v", err)
+	}
+	if p := fb.GetRootAsProfile(payload, 0); p.Banner(nil) != nil {
+		t.Error("ineligible profile unexpectedly carries a banner block")
+	}
+}