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.

server/src/utils/mailer.js

@@ -14,6 +14,7 @@ const nodemailer = require('nodemailer')
 const emailConfig = require('../model/emailConfig/emailConfig.model')
 const authProviders = require('../model/authProviders/authProviders.model')
 const settings = require('../model/settings/settings.model')
+const brand = require('../config/brand')
 const log = require('./logger')('mailer')
 
 // Ready to send only when enabled, connected (has a refresh token), and we know
@@ -76,7 +77,7 @@ async function sendContactMessage({ name, email, message }) {
       from: fromHeader(config),
       to,
       replyTo: email,
-      subject: `UOMysticmoon contact from ${name || 'a visitor'}`,
+      subject: `${brand.name} contact from ${name || 'a visitor'}`,
       text: `From: ${name || 'unknown'} <${email || 'no email'}>\n\n${message}`,
     })
     await emailConfig.recordStatus({ status: 'connected', statusDetail: 'Last send OK', lastVerifiedAt: new Date() })
@@ -110,7 +111,7 @@ async function sendTest(to) {
     await transport.sendMail({
       from: fromHeader(config),
       to: recipient,
-      subject: 'UOMysticmoon email test',
+      subject: `${brand.name} email test`,
       text: 'This is a test message confirming Gmail OAuth2 email delivery is working.',
     })
     await emailConfig.recordStatus({ status: 'connected', statusDetail: 'Test send OK', lastVerifiedAt: new Date() })
@@ -139,9 +140,9 @@ async function sendInvite({ to, acceptUrl, role, invitedByName }) {
     await transport.sendMail({
       from: fromHeader(config),
       to,
-      subject: 'Your UOMysticmoon invitation',
+      subject: `Your ${brand.name} invitation`,
       text:
-        `You have been invited${by} to join UOMysticmoon${roleLabel}.\n\n` +
+        `You have been invited${by} to join ${brand.name}${roleLabel}.\n\n` +
         `Accept your invitation and set up your account here:\n${acceptUrl}\n\n` +
         `This link is single-use and will expire. If you weren't expecting this, you can ignore it.`,
     })