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).

pkg/fbs/scrabble.fbs

@@ -180,10 +180,20 @@ table Ack {
 // BannerCampaign is one campaign in the rotation feed: a GCD-reduced show weight
 // (the client runs a smooth weighted round-robin over campaigns by this weight)
 // and its messages in display order, each already resolved to the viewer's bot
-// language (the stored en/ru pair is picked server-side).
+// language (the stored en/ru pair is picked server-side). The override_* colours
+// are an optional per-campaign palette: override_bg/fg/link paint the strip on
+// every theme, and the *_dark trio further overrides the dark theme (the client
+// resolves dark ← dark ?? all ?? token, light ← all ?? token). Empty means the
+// campaign keeps the neutral theme tokens (all added trailing — backward-compatible).
 table BannerCampaign {
   weight:int;
   messages:[string];
+  override_bg:string;
+  override_fg:string;
+  override_link:string;
+  override_bg_dark:string;
+  override_fg_dark:string;
+  override_link_dark:string;
 }
 
 // BannerInfo is the advertising-banner block of an eligible viewer's profile: the