ci: harden the qdrant integration job

The integration test job has been failing intermittently on main and on
open pull requests with `httpx.ReadTimeout: timed out` on the first write
to Qdrant, while `GET /collections` and `/healthz` kept answering.

Cause: the qdrant-client default request timeout is 5 seconds, and the job
only waits for `/healthz`, which is served before the storage engine
accepts writes. On a shared runner doing several builds at once, the first
collection creation or indexing exceeds 5s and the whole fixture errors
out.

- pin qdrant/qdrant to v1.19 (the digest `latest` and the cluster already
  run) and let Renovate track the pin in the workflows as well
- wait for a real write round-trip (create + delete a probe collection)
  instead of `/healthz` alone, and dump the container logs if it never
  becomes writable
- make the Qdrant client timeout configurable via QDRANT_TIMEOUT and set
  it to 60s in CI, where there is no latency SLA

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   qdrant:
-    image: docker.io/qdrant/qdrant:latest
+    image: docker.io/qdrant/qdrant:v1.19
     container_name: mcp_maildir_qdrant
     ports:
       - "127.0.0.1:6333:6333" # REST API