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/contexts/SiteContext.jsx

@@ -23,13 +23,24 @@ export function SiteProvider({ children }) {
     refresh()
   }, [refresh])
 
+  const brand = settings.brand || {}
+
+  // Apply the instance accent color to the CSS variable the theme is built on,
+  // so branding flows to every `var(--accent)` at runtime (no rebuild).
+  useEffect(() => {
+    if (brand.accent) document.documentElement.style.setProperty('--accent', brand.accent)
+  }, [brand.accent])
+
   const value = {
     settings,
     loading,
     refresh,
+    brand,
     mode: settings.site_mode || 'live',
-    siteTitle: settings.site_title || 'UOMysticmoon',
-    contactEmail: settings.contact_email || 'UOMysticmoon@gmail.com',
+    siteTitle: brand.name || settings.site_title || 'Runic Gateway',
+    siteShortName: brand.shortName || brand.name || settings.site_title || 'Runic Gateway',
+    contactEmail: brand.contactEmail || settings.contact_email || '',
+    heroImage: brand.hero || '/assets/img/hero-moon.png',
   }
 
   return <SiteContext.Provider value={value}>{children}</SiteContext.Provider>