feat: add AI summary / transcript integration for recordings

Ported from greenlight-tm: when TRANSCRIPTION_API_URL and
TRANSCRIPTION_API_KEY are set in the .env and the per-room
"AI summary / transcript" setting is enabled, Redlight POSTs to
${TRANSCRIPTION_API_URL}/prompt (header X-Api-Key) as soon as the
"video" format of a recording becomes available.

- transcription.js: service with env gating, timeout and logging
- bbb.js: pass meta_bbb-recording-ready-url on meeting create when the
  room has the setting enabled
- recordings.js: unauthenticated /recording-ready callback that
  verifies BBB's signed_parameters JWT (HS256, shared secret), caches
  the recording and requests the transcription once per recording
  (deduped via transcript_requested_at)
- rooms: new recording_transcript column, accepted on create/update
- branding endpoint exposes transcriptionEnabled so the UI only shows
  the toggle when the server is configured
- RoomDetail: toggle under "Allow recording" (disabled when recording
  is off), de/en i18n
- chore: bump version to 2.4.0

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/routes/federation.js

@@ -41,7 +41,7 @@ export function wellKnownHandler(req, res) {
         federation_api: '/api/federation',
         public_key: getPublicKey(),
         software: 'Redlight',
-        version: '2.3.0',
+        version: '2.4.0',
     });
 }