Fix Lyria API response parsing — audio in parts[1], not parts[0]

The Lyria API returns a candidates[].content.parts[] array where:
- parts[0] = text metadata (e.g. '<instrumental>')
- parts[1] = inlineData with the actual audio payload

Previous code only checked parts[0] for inlineData, which always failed
with 'No audio data in response'. Fixed by iterating all parts to find
the inlineData entry.

Successfully generated first real audio clip (lo-fi hip hop, 30s, 728KB MP3).

Also includes: .env.example, requirements updates, docker-compose fixes,
frontend vite config, logger setup.

backend/requirements.txt

@@ -2,5 +2,6 @@ fastapi==0.115.0
 uvicorn[standard]==0.32.0
 httpx==0.27.0
 aiosqlite==0.20.0
-pydantic==2.9.0
+pydantic>=2.10.0
 python-multipart==0.0.12
+python-dotenv==1.0.0