feat(brand): BRAND_* env scheme — instance branding without a rebuild

Replace baked-in UOM/MysticMoon/UOMysticmoon branding with a BRAND_* env
scheme so one prebuilt image runs as any shard; UOMysticmoon becomes the
first tenant that sets these vars rather than a special case in the code.

Architecture (chosen because the app ships as a prebuilt image):
- server/src/config/brand.js + bot/src/brand.js read BRAND_* once at boot,
  with Runic Gateway defaults.
- Text/colors reach the SPA at RUNTIME through the existing public settings
  API (settings.model.getPublic -> SiteContext), so no client rebuild. The
  admin-editable site title + contact email still override BRAND_NAME/email.
- SiteContext applies BRAND_ACCENT_COLOR to the --accent CSS var at runtime.
- Express templates the built index.html <title>/description/OG/favicon at
  serve time from BRAND_* (renderIndexHtml in app.js).
- Server-side consumers read brand directly: emails, TOTP issuer, API docs,
  boot logs, HTML error page. Bot uses it for embed color + logs.

Assets: logo/hero/favicon delivered from a ./brand:/app/brand bind-mount
(BRAND_LOGO/HERO/FAVICON), with neutral defaults baked in; hero falls back
to a built-in image when unset.

Scope: also genericized package.json names (uomysticmoon-* -> runic-gateway-*)
and the DB_NAME/DB_USER/COOKIE_NAME code defaults (runic_gateway/runic/
rg_token). Production keeps its real values by pinning them in .env — see
.env.uomysticmoon.example, which reproduces the exact UOMysticmoon identity
(proof the substitution works). Changing a deployed COOKIE_NAME invalidates
existing sessions, so UOMysticmoon pins uomm_token.

Verified: 193 server tests pass, client builds, app.js loads + templates the
built index.html, brand transform injects title/description/OG/favicon.

client/src/routes/public/Maintenance.jsx

@@ -2,14 +2,12 @@ import { Link } from 'react-router-dom'
 import MoonDot from '../../components/MoonDot.jsx'
 import { useSite } from '../../contexts/SiteContext.jsx'
 
-const HERO_BG =
-  "linear-gradient(180deg,rgba(11,15,20,0.55) 0%,rgba(11,15,20,0.74) 60%,rgba(11,15,20,0.92) 100%),url('/assets/img/uomysticmoon-main-hero.png')"
-
 export default function Maintenance() {
-  const { settings, contactEmail } = useSite()
+  const { settings, contactEmail, siteShortName, heroImage } = useSite()
+  const heroBg = `linear-gradient(180deg,rgba(11,15,20,0.55) 0%,rgba(11,15,20,0.74) 60%,rgba(11,15,20,0.92) 100%),url('${heroImage}')`
   const message =
     settings.maintenance_message ||
-    'Mysticmoon is in maintenance while we shape its towns, roads, and dungeons. The gates will open soon. Until then, follow along as the world wakes.'
+    `${siteShortName} is in maintenance while we shape its towns, roads, and dungeons. The gates will open soon. Until then, follow along as the world wakes.`
 
   return (
     <main
@@ -22,7 +20,7 @@ export default function Maintenance() {
         padding: '80px max(18px,calc((100% - 760px)/2))',
         overflow: 'hidden',
         backgroundColor: 'var(--bg-deep)',
-        backgroundImage: HERO_BG,
+        backgroundImage: heroBg,
         backgroundPosition: 'center',
         backgroundRepeat: 'no-repeat',
         backgroundSize: 'cover',