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/mailer.js

@@ -64,7 +64,6 @@ export async function sendVerificationEmail(to, name, verifyUrl, appName = 'Redl
   const from = process.env.SMTP_FROM || process.env.SMTP_USER;
   const headerAppName = sanitizeHeaderValue(appName);
   const safeName = escapeHtml(name);
-  const safeAppName = escapeHtml(appName);
 
   await transporter.sendMail({
     from: `"${headerAppName}" <${from}>`,
@@ -112,7 +111,6 @@ export async function sendFederationInviteEmail(to, name, fromUser, roomName, me
   const safeFromUser = escapeHtml(fromUser);
   const safeRoomName = escapeHtml(roomName);
   const safeMessage = message ? escapeHtml(message) : null;
-  const safeAppName = escapeHtml(appName);
 
   const introHtml = t(lang, 'email.federationInvite.intro')
     .replace('{fromUser}', `<strong style="color:#cdd6f4;">${safeFromUser}</strong>`);