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/calendar.js

@@ -4,6 +4,7 @@ import { getDb } from '../config/database.js';
 import { authenticateToken, getBaseUrl } from '../middleware/auth.js';
 import { log } from '../config/logger.js';
 import { sendCalendarInviteEmail } from '../config/mailer.js';
+import { getAppName } from '../config/appName.js';
 import {
   isFederationEnabled,
   getFederationDomain,
@@ -219,7 +220,7 @@ router.put('/events/:id', authenticateToken, async (req, res) => {
         end_time = COALESCE(?, end_time),
         room_uid = ?,
         color = COALESCE(?, color),
-        reminder_minutes = COALESCE(?, reminder_minutes),
+        reminder_minutes = ?,
         reminder_sent_at = ${resetReminder ? 'NULL' : 'reminder_sent_at'},
         updated_at = CURRENT_TIMESTAMP
       WHERE id = ?
@@ -230,7 +231,9 @@ router.put('/events/:id', authenticateToken, async (req, res) => {
       end_time || null,
       room_uid !== undefined ? (room_uid || null) : event.room_uid,
       color || null,
-      validReminder !== undefined ? validReminder : null,
+      // Not in payload → keep existing; present (incl. null) → set/clear directly.
+      // COALESCE can't be used here: it would treat an explicit null as "keep".
+      validReminder !== undefined ? validReminder : event.reminder_minutes,
       req.params.id,
     ]);
 
@@ -323,7 +326,7 @@ router.post('/events/:id/share', authenticateToken, async (req, res) => {
     if (targetUser?.email) {
       const appUrl = getBaseUrl(req);
       const inboxUrl = `${appUrl}/federation/inbox`;
-      const appName = process.env.APP_NAME || 'Redlight';
+      const appName = await getAppName();
       const senderUser = await db.get('SELECT name, display_name FROM users WHERE id = ?', [req.user.id]);
       const fromDisplay = (senderUser?.display_name && senderUser.display_name !== '') ? senderUser.display_name : (senderUser?.name || req.user.name);
       sendCalendarInviteEmail(
@@ -645,7 +648,7 @@ router.post(['/receive-event', '/calendar-event'], calendarFederationLimiter, as
     if (targetUser.email) {
       const appUrl = getBaseUrl(req);
       const inboxUrl = `${appUrl}/federation/inbox`;
-      const appName = process.env.APP_NAME || 'Redlight';
+      const appName = await getAppName();
       sendCalendarInviteEmail(
         targetUser.email, targetUser.name, from_user,
         title, start_time, end_time, description || null,