feat(banner): per-campaign colour overrides and urgent alerts

Non-default campaigns gain an optional colour override (background / text /
link) in two sets — one for every theme, one for the dark theme only — and an
"urgent" flag.

- Colours ride profile.get as six trailing FlatBuffers strings on
  BannerCampaign (backward-compatible). The client resolves the cascade
  (dark <- dark ?? all, light <- all) per rendered theme and derives the strip
  border from the background in JS (no CSS color-mix, for the old Android
  WebView floor); AdBanner applies them as inline vars scoped to the strip.
- Urgent is resolved entirely server-side: while any enabled, in-window urgent
  campaign exists, computeActiveSet returns only the urgent campaigns and
  bannerFor skips the eligibility gate — so a system notice reaches every viewer
  (paid / hint-holding / no_banner included) and preempts the ordinary feed. No
  wire field; it appears on each viewer's next profile.get.
- Admin console (/_gm/banners): native colour pickers + a live light/dark
  preview of the strip, and an urgent toggle. The default campaign stays plain,
  enforced by the service and a DB CHECK.

Migration 00009 is additive (nullable colour columns + a bool default +
all-or-nothing / hex / default-plain CHECKs) — expand-contract, rollback-safe.

Docs: ARCHITECTURE §10, UI_DESIGN, FUNCTIONAL (+ru). Tests: ads unit (urgent
preempt + colour validation), codec + resolver unit, gateway transcode, and
integration (colour round-trip + urgent bypass against real Postgres).

backend/internal/ads/ads_test.go

@@ -46,12 +46,19 @@ func TestComputeActiveSet(t *testing.T) {
 	timed := func(name string, weight int, starts, ends *time.Time, msgs []Message) Campaign {
 		return Campaign{Name: name, Weight: weight, Enabled: true, StartsAt: starts, EndsAt: ends, Messages: msgs}
 	}
+	urgent := func(name string, weight int, msgs []Message) Campaign {
+		c := timed(name, weight, nil, nil, msgs)
+		c.Urgent = true
+		return c
+	}
+	red := &ColorSet{Bg: "#aa0000", Fg: "#ffffff", Link: "#ffdd00"}
 
 	tests := []struct {
-		name      string
-		campaigns []Campaign
-		lang      string
-		want      []ActiveCampaign
+		name       string
+		campaigns  []Campaign
+		lang       string
+		want       []ActiveCampaign
+		wantUrgent bool
 	}{
 		{
 			name:      "default only reduces to weight 1",
@@ -152,22 +159,71 @@ func TestComputeActiveSet(t *testing.T) {
 			lang: "en",
 			want: []ActiveCampaign{{Weight: 1, Messages: []string{"one-en", "two-en"}}},
 		},
+		{
+			name: "urgent preempts default and normal timed",
+			campaigns: []Campaign{
+				def(msg("house")),
+				timed("promo", 40, nil, nil, msg("promo")),
+				urgent("alert", 50, msg("alert")),
+			},
+			lang: "en",
+			// only the urgent campaign survives; a lone weight reduces to 1.
+			want:       []ActiveCampaign{{Weight: 1, Messages: []string{"alert-en"}}},
+			wantUrgent: true,
+		},
+		{
+			name: "multiple urgent share the feed by gcd",
+			campaigns: []Campaign{
+				def(msg("house")),
+				urgent("a", 60, msg("a")),
+				urgent("b", 40, msg("b")),
+			},
+			lang: "en",
+			// default and any non-urgent dropped; gcd(60,40)=20 -> 3 and 2.
+			want: []ActiveCampaign{
+				{Weight: 3, Messages: []string{"a-en"}},
+				{Weight: 2, Messages: []string{"b-en"}},
+			},
+			wantUrgent: true,
+		},
+		{
+			name: "out-of-window urgent does not preempt",
+			campaigns: []Campaign{
+				def(msg("house")),
+				func() Campaign { c := urgent("future", 50, msg("future")); c.StartsAt = &future; return c }(),
+			},
+			lang: "en",
+			// the urgent campaign is not yet live, so the normal default feed stands.
+			want: []ActiveCampaign{{Weight: 1, Messages: []string{"house-en"}}},
+		},
+		{
+			name: "colour overrides ride the active campaign",
+			campaigns: []Campaign{
+				def(msg("house")),
+				func() Campaign { c := timed("promo", 100, nil, nil, msg("promo")); c.OverrideAll = red; return c }(),
+			},
+			lang: "en",
+			want: []ActiveCampaign{{Weight: 1, Messages: []string{"promo-en"}, OverrideAll: red}},
+		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			got := computeActiveSet(tt.campaigns, now, tt.lang)
+			got, gotUrgent := computeActiveSet(tt.campaigns, now, tt.lang)
 			if !reflect.DeepEqual(got, tt.want) {
 				t.Errorf("computeActiveSet() =\n  %#v\nwant\n  %#v", got, tt.want)
 			}
+			if gotUrgent != tt.wantUrgent {
+				t.Errorf("computeActiveSet() urgent = %v, want %v", gotUrgent, tt.wantUrgent)
+			}
 		})
 	}
 }
 
 func TestComputeActiveSetEmpty(t *testing.T) {
 	// No campaigns at all yields an empty (non-nil-or-nil) feed without panicking.
-	if got := computeActiveSet(nil, time.Now(), "en"); len(got) != 0 {
-		t.Errorf("computeActiveSet(nil) = %#v, want empty", got)
+	if got, urgent := computeActiveSet(nil, time.Now(), "en"); len(got) != 0 || urgent {
+		t.Errorf("computeActiveSet(nil) = %#v urgent=%v, want empty non-urgent", got, urgent)
 	}
 }