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/i18n/de.json

@@ -42,5 +42,9 @@
       "note": "Der Termin wurde automatisch aus deinem Kalender entfernt.",
       "footer": "Diese Nachricht wurde automatisch von {appName} versendet."
     }
+  },
+  "bbb": {
+    "defaultWelcome": "Willkommen im Meeting!",
+    "inviteHint": "Lade weitere Teilnehmer ein, indem du diesen Link teilst:"
   }
 }

server/i18n/en.json

@@ -42,5 +42,9 @@
       "note": "The event has been automatically removed from your calendar.",
       "footer": "This message was sent automatically by {appName}."
     }
+  },
+  "bbb": {
+    "defaultWelcome": "Welcome to the meeting!",
+    "inviteHint": "To invite other participants, share this link:"
   }
 }