Add QDRANT_API_KEY support to rag-service

- Add QDRANT_API_KEY to config.py (empty string = no auth)
- Pass api_key to QdrantClient constructor (None when empty)
- Add QDRANT_API_KEY to coolify compose and env example

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

.env.coolify.example

@@ -26,6 +26,7 @@ S3_SECURE=true
 
 # --- External Qdrant (Coolify-managed) ---
 QDRANT_URL=http://<coolify-qdrant-hostname>:6333
+QDRANT_API_KEY=
 
 # --- SMTP (Real mail server) ---
 SMTP_HOST=smtp.example.com