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).

gateway/internal/backendclient/api.go

@@ -53,9 +53,18 @@ type BannerResp struct {
 
 // BannerCampaignResp is one campaign in the rotation feed: a GCD-reduced show
 // weight and its messages, in display order, already resolved to one language.
+// The override_* colours are the optional per-campaign palette (empty when the
+// campaign keeps the neutral theme tokens); they ride through to the client
+// verbatim, mirroring the backend DTO.
 type BannerCampaignResp struct {
-	Weight   int      `json:"weight"`
-	Messages []string `json:"messages"`
+	Weight           int      `json:"weight"`
+	Messages         []string `json:"messages"`
+	OverrideBg       string   `json:"override_bg,omitempty"`
+	OverrideFg       string   `json:"override_fg,omitempty"`
+	OverrideLink     string   `json:"override_link,omitempty"`
+	OverrideBgDark   string   `json:"override_bg_dark,omitempty"`
+	OverrideFgDark   string   `json:"override_fg_dark,omitempty"`
+	OverrideLinkDark string   `json:"override_link_dark,omitempty"`
 }
 
 // BannerTimingsResp mirrors the backend's global display timings.