fix: resolve server bugs and unify app-name handling

Bug fixes:
- bbb.js: replace undefined t('defaultWelcome') call that threw a
  ReferenceError when a room had an empty welcome message, breaking
  meeting creation. Default welcome and the guest-invite hint are now
  localised via the i18n system (new "bbb" namespace in de/en).
- auth.js: app name was read from the never-written 'branding' settings
  key, so custom names never appeared in verification emails or the TOTP
  issuer. Now resolved through a shared getAppName() helper.
- auth.js: lowercase the email in the registration duplicate check so
  case-variant duplicates return a clean 409 instead of a 500 (UNIQUE
  violation).
- federation.js: select the user's "language" column so federation
  invite emails respect the recipient's language instead of always
  defaulting to English.
- calendar.js: a set reminder could not be cleared. COALESCE treated an
  explicit reminder_minutes: null as "keep existing"; use a direct
  assignment that distinguishes "omitted" (keep) from "null" (clear).
- index.js / analytics.js: exclude the BBB learning-analytics callback
  from the global 100kb body limit and give it its own 5mb limit, since
  analytics payloads for large meetings can be several MB.

Cleanup:
- Add server/config/appName.js as the single source of truth for the
  app name (admin setting -> APP_NAME env -> 'Redlight') and use it in
  auth, admin, rooms, calendar and federation, replacing the previous
  mix of wrong DB key, direct app_name reads and bare process.env reads.
- Localise the BBB default welcome message in the room owner's language.
- Remove two unused safeAppName variables in mailer.js.

server/index.js

@@ -51,7 +51,14 @@ const corsOptions = process.env.APP_URL
   ? { origin: process.env.APP_URL, credentials: true }
   : { origin: false };
 app.use(cors(corsOptions));
-app.use(express.json({ limit: '100kb' }));
+// Global JSON body limit kept tight as a hardening measure. The BBB learning
+// analytics callback can send much larger payloads, so it is excluded here and
+// gets its own, more generous limit on the route itself (see routes/analytics.js).
+const jsonParser = express.json({ limit: '100kb' });
+app.use((req, res, next) => {
+  if (req.path.startsWith('/api/analytics/callback/')) return next();
+  return jsonParser(req, res, next);
+});
 // Request/Response logging (filters sensitive fields)
 app.use(requestResponseLogger);