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/routes/rooms.js

@@ -9,6 +9,7 @@ import { authenticateToken, getBaseUrl } from '../middleware/auth.js';
 import { log } from '../config/logger.js';
 import { createNotification } from '../config/notifications.js';
 import { sendGuestInviteEmail } from '../config/mailer.js';
+import { getAppName } from '../config/appName.js';
 import {
   createMeeting,
   joinMeeting,
@@ -513,7 +514,9 @@ router.post('/:uid/start', authenticateToken, async (req, res) => {
     const analyticsCallbackURL = room.learning_analytics
       ? `${baseUrl}/api/analytics/callback/${room.uid}?token=${getAnalyticsToken(room.uid)}`
       : null;
-    await createMeeting(room, baseUrl, loginURL, presentationUrl, analyticsCallbackURL);
+    // Localise BBB's default welcome message in the room owner's language
+    const owner = await db.get('SELECT language FROM users WHERE id = ?', [room.user_id]);
+    await createMeeting(room, baseUrl, loginURL, presentationUrl, analyticsCallbackURL, owner?.language || 'en');
     const avatarURL = getUserAvatarURL(req, req.user);
     const displayName = req.user.display_name || req.user.name;
     const joinUrl = await joinMeeting(room.uid, displayName, true, avatarURL);
@@ -662,7 +665,9 @@ router.post('/:uid/guest-join', guestJoinLimiter, async (req, res) => {
       const analyticsCallbackURL = room.learning_analytics
         ? `${baseUrl}/api/analytics/callback/${room.uid}?token=${getAnalyticsToken(room.uid)}`
         : null;
-      await createMeeting(room, baseUrl, loginURL, null, analyticsCallbackURL);
+      // Localise BBB's default welcome message in the room owner's language
+      const owner = await db.get('SELECT language FROM users WHERE id = ?', [room.user_id]);
+      await createMeeting(room, baseUrl, loginURL, null, analyticsCallbackURL, owner?.language || 'en');
     }
 
     // Check moderator code
@@ -908,7 +913,7 @@ router.post('/invite-email', authenticateToken, async (req, res) => {
       ? `${baseUrl}/join/${room.uid}?ac=${encodeURIComponent(room.access_code)}`
       : `${baseUrl}/join/${room.uid}`;
 
-    const appName = process.env.APP_NAME || 'Redlight';
+    const appName = await getAppName();
     const fromUser = req.user.display_name || req.user.name;
     const lang = req.user.language || 'en';