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).

docs/FUNCTIONAL.md

@@ -485,8 +485,19 @@ through**, regardless of their interface language. The client rotates the eligib
 **fairly** — every campaign gets its weighted share each cycle, evenly spread rather than at random —
 and a campaign with several messages shows them in turn.
 
+A non-default campaign can carry its own **colours** — background, text and link — so a sponsored or
+operational message stands out from the neutral strip. Colours come in two sets: one that applies on
+**every theme**, and an optional second that overrides the **dark theme** only, so a campaign reads
+well on both; the strip's border is derived from the background automatically. A campaign can also be
+marked **urgent**: an urgent campaign is shown to **everyone** — even paid players, hint holders and
+`no_banner` users — and, while it is live, it is the **only** thing the strip shows (ordinary
+campaigns and the house default step aside). Urgent is for genuine system notices (maintenance,
+outages), not advertising.
+
 Operators manage all of this in the admin console at **`/_gm/banners`**: create, edit, enable/disable
-and schedule campaigns, write each campaign's bilingual messages (reorder or remove them), and set
-the global display **timings** (how long a message holds, the scroll of an over-long message, and the
-fade-out → gap → fade-in transition between messages). The default campaign cannot be deleted and
-keeps at least one message.
+and schedule campaigns, write each campaign's bilingual messages (reorder or remove them), pick a
+non-default campaign's optional override colours (a native colour picker with a live light-and-dark
+preview of the strip) and mark it urgent, and set the global display **timings** (how long a message
+holds, the scroll of an over-long message, and the fade-out → gap → fade-in transition between
+messages). The default campaign cannot be deleted, keeps at least one message, and stays plain (no
+colours, never urgent).