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>

docker-compose.coolify.yml

@@ -143,6 +143,7 @@ services:
     environment:
       PORT: 8097
       QDRANT_URL: ${QDRANT_URL}
+      QDRANT_API_KEY: ${QDRANT_API_KEY:-}
       MINIO_ENDPOINT: ${S3_ENDPOINT}
       MINIO_ACCESS_KEY: ${S3_ACCESS_KEY}
       MINIO_SECRET_KEY: ${S3_SECRET_KEY}