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>

rag-service/config.py

@@ -6,6 +6,7 @@ class Settings:
 
     # Qdrant
     QDRANT_URL: str = os.getenv("QDRANT_URL", "http://localhost:6333")
+    QDRANT_API_KEY: str = os.getenv("QDRANT_API_KEY", "")
 
     # MinIO
     MINIO_ENDPOINT: str = os.getenv("MINIO_ENDPOINT", "localhost:9000")