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/Portal.jsx

@@ -9,10 +9,10 @@ import { defaultLayout, parseLayout, heroBackground } from '../../lib/heroLayout
 const PREVIEW = typeof window !== 'undefined' && new URLSearchParams(window.location.search).get('preview') === '1'
 
 export default function Portal() {
-  const { settings } = useSite()
+  const { settings, siteShortName, siteTitle, heroImage } = useSite()
   const teaser =
     settings.homepage_teaser ||
-    'Mysticmoon is still being shaped beneath a midnight sky — a quiet preview for the news, screenshots, guides, and community notes to come as the world wakes.'
+    `${siteShortName} is still being shaped beneath a midnight sky — a quiet preview for the news, screenshots, guides, and community notes to come as the world wakes.`
 
   // Published layout (public). Falls back to the pre-populated default if missing,
   // malformed, the wrong version, or empty — so the hero is never blank.
@@ -34,9 +34,9 @@ export default function Portal() {
   }, [])
 
   const active = (PREVIEW && draft) || (published && published.elements.length ? published : null)
-  const layout = active || defaultLayout(teaser)
+  const layout = active || defaultLayout(teaser, siteTitle)
   const isDefault = !active
-  const bgStyle = heroBackground(layout, { isDefault })
+  const bgStyle = heroBackground(layout, { isDefault, defaultImage: heroImage })
   const elements = [...layout.elements].sort((a, b) => (a.z || 0) - (b.z || 0))
 
   return (