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/service.go

@@ -3,6 +3,7 @@ package ads
 import (
 	"context"
 	"fmt"
+	"regexp"
 	"strings"
 	"time"
 
@@ -40,17 +41,20 @@ func NewService(store *Store) *Service { return &Service{store: store} }
 // ActiveSet returns the resolved rotation feed for a viewer in language lang
 // (en/ru) together with the global display timings: the currently-active
 // campaigns, each with its GCD-reduced show weight and its messages resolved to
-// lang, ready for the client's weighted round-robin.
-func (s *Service) ActiveSet(ctx context.Context, lang string) ([]ActiveCampaign, Timings, error) {
+// lang, ready for the client's weighted round-robin. The bool result reports
+// whether the feed is urgent — an urgent feed is shown to every viewer
+// regardless of eligibility (the caller skips the eligibility gate for it).
+func (s *Service) ActiveSet(ctx context.Context, lang string) ([]ActiveCampaign, Timings, bool, error) {
 	campaigns, err := s.store.ActiveCampaigns(ctx)
 	if err != nil {
-		return nil, Timings{}, err
+		return nil, Timings{}, false, err
 	}
 	timings, err := s.store.Settings(ctx)
 	if err != nil {
-		return nil, Timings{}, err
+		return nil, Timings{}, false, err
 	}
-	return computeActiveSet(campaigns, time.Now().UTC(), lang), timings, nil
+	set, urgent := computeActiveSet(campaigns, time.Now().UTC(), lang)
+	return set, timings, urgent, nil
 }
 
 // ListCampaigns returns every campaign with its messages, for the admin console.
@@ -76,8 +80,13 @@ func (s *Service) CreateCampaign(ctx context.Context, c Campaign) (uuid.UUID, er
 	if err := validWindow(c.StartsAt, c.EndsAt); err != nil {
 		return uuid.Nil, err
 	}
+	all, dark, err := validOverrides(c.OverrideAll, c.OverrideDark)
+	if err != nil {
+		return uuid.Nil, err
+	}
 	return s.store.CreateCampaign(ctx, Campaign{
 		Name: name, Weight: c.Weight, Enabled: c.Enabled, StartsAt: c.StartsAt, EndsAt: c.EndsAt,
+		OverrideAll: all, OverrideDark: dark, Urgent: c.Urgent,
 	})
 }
 
@@ -99,6 +108,7 @@ func (s *Service) UpdateCampaign(ctx context.Context, c Campaign) error {
 		upd.Enabled = true
 		upd.StartsAt = nil
 		upd.EndsAt = nil
+		// The default (house) campaign stays plain: no colour overrides, never urgent.
 	} else {
 		if err := validWeight(c.Weight); err != nil {
 			return err
@@ -106,10 +116,17 @@ func (s *Service) UpdateCampaign(ctx context.Context, c Campaign) error {
 		if err := validWindow(c.StartsAt, c.EndsAt); err != nil {
 			return err
 		}
+		all, dark, err := validOverrides(c.OverrideAll, c.OverrideDark)
+		if err != nil {
+			return err
+		}
 		upd.Weight = c.Weight
 		upd.Enabled = c.Enabled
 		upd.StartsAt = c.StartsAt
 		upd.EndsAt = c.EndsAt
+		upd.OverrideAll = all
+		upd.OverrideDark = dark
+		upd.Urgent = c.Urgent
 	}
 	return s.store.UpdateCampaign(ctx, upd)
 }
@@ -254,6 +271,46 @@ func validWindow(starts, ends *time.Time) error {
 	return nil
 }
 
+// hexColor matches a "#rrggbb" colour — the format the console's native colour
+// input emits and the wire carries.
+var hexColor = regexp.MustCompile(`^#[0-9a-fA-F]{6}$`)
+
+// validOverrides validates the two optional colour sets together and returns
+// their normalised copies (nil when a set is absent), so a caller can store them
+// directly.
+func validOverrides(all, dark *ColorSet) (*ColorSet, *ColorSet, error) {
+	va, err := validColorSet(all)
+	if err != nil {
+		return nil, nil, err
+	}
+	vd, err := validColorSet(dark)
+	if err != nil {
+		return nil, nil, err
+	}
+	return va, vd, nil
+}
+
+// validColorSet validates one optional colour override: a nil set passes (no
+// override); otherwise all three colours must be present and "#rrggbb". It
+// returns a trimmed, lower-cased copy.
+func validColorSet(cs *ColorSet) (*ColorSet, error) {
+	if cs == nil {
+		return nil, nil
+	}
+	bg := strings.TrimSpace(cs.Bg)
+	fg := strings.TrimSpace(cs.Fg)
+	link := strings.TrimSpace(cs.Link)
+	if bg == "" || fg == "" || link == "" {
+		return nil, fmt.Errorf("%w: a colour override needs all of background, text and link", ErrValidation)
+	}
+	for _, h := range []string{bg, fg, link} {
+		if !hexColor.MatchString(h) {
+			return nil, fmt.Errorf("%w: colours must be #rrggbb hex", ErrValidation)
+		}
+	}
+	return &ColorSet{Bg: strings.ToLower(bg), Fg: strings.ToLower(fg), Link: strings.ToLower(link)}, nil
+}
+
 // validBodies trims and bounds both mandatory language bodies of a message.
 func validBodies(bodyEn, bodyRu string) (string, string, error) {
 	en := strings.TrimSpace(bodyEn)