feat: add audios table and ElevenLabs TTS endpoint

- New audios table with voice params, S3 link, alignment JSON
- POST /api/audios/generate calls ElevenLabs with-timestamps, uploads to S3
- GET/PATCH/DELETE /api/audios endpoints
- Requires ELEVENLABS_API_KEY env var

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -11,3 +11,6 @@ PORT=3000
 
 # Anthropic (für Auto-Pairs-Generierung)
 ANTHROPIC_API_KEY=sk-ant-...
+
+# ElevenLabs (für TTS-Generierung)
+ELEVENLABS_API_KEY=sk_...