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.

bot/src/discord/newsAnnounce.js

@@ -4,6 +4,7 @@
 const { EmbedBuilder } = require('discord.js')
 
 const guildConfig = require('../model/guildConfig')
+const brand = require('../brand')
 const createLogger = require('../utils/logger')
 
 const log = createLogger('news')
@@ -15,7 +16,7 @@ async function postAnnounce(client, guildId, { title, excerpt, url, imageUrl })
   const channel = await client.channels.fetch(channelId)
   if (!channel || !channel.isTextBased()) throw new Error('Configured news channel is missing or not text-based.')
 
-  const embed = new EmbedBuilder().setColor(0x6a8fc2).setTitle(title).setURL(url)
+  const embed = new EmbedBuilder().setColor(brand.accentInt).setTitle(title).setURL(url)
   if (excerpt) embed.setDescription(excerpt)
   if (imageUrl) embed.setImage(imageUrl)