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/admin.js

@@ -4,6 +4,7 @@ import { v4 as uuidv4 } from 'uuid';
 import { getDb } from '../config/database.js';
 import { authenticateToken, requireAdmin, getBaseUrl } from '../middleware/auth.js';
 import { isMailerConfigured, sendInviteEmail } from '../config/mailer.js';
+import { getAppName } from '../config/appName.js';
 import { log } from '../config/logger.js';
 import {
   getOAuthConfig,
@@ -217,9 +218,8 @@ router.post('/invites', authenticateToken, requireAdmin, async (req, res) => {
     const baseUrl = getBaseUrl(req);
     const inviteUrl = `${baseUrl}/register?invite=${token}`;
 
-    // Load app name
-    const brandingSetting = await db.get("SELECT value FROM settings WHERE key = 'app_name'");
-    const appName = brandingSetting?.value || 'Redlight';
+    // Load configured app name (admin setting → env → default)
+    const appName = await getAppName();
 
     if (isMailerConfigured()) {
       try {