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/config/appName.js

@@ -0,0 +1,20 @@
+import { getDb } from './database.js';
+
+/**
+ * Resolve the configured application name.
+ * Resolution order: admin-set 'app_name' setting → APP_NAME env var → 'Redlight'.
+ *
+ * The app name is stored in the settings table under the key 'app_name'
+ * (see routes/branding.js). This helper is the single source of truth so the
+ * configured name is used consistently across emails, the 2FA issuer, etc.
+ */
+export async function getAppName() {
+  try {
+    const db = getDb();
+    const row = await db.get("SELECT value FROM settings WHERE key = 'app_name'");
+    if (row?.value) return row.value;
+  } catch {
+    // fall through to env/default if the DB is unavailable
+  }
+  return process.env.APP_NAME || 'Redlight';
+}