perf(monitoring): stop blocking event loop with synchronous Docker calls

Customer search and detail loads were intermittently slow because every
customer-table render (including each search keystroke) triggered
/monitoring/customers/local-update-status, which looped synchronously over
all customers doing blocking `docker inspect` subprocess calls on the event
loop — stalling all other in-flight requests, including search itself.

- Offload per-service image/container inspection to the thread pool and run
  checks concurrently instead of sequentially (image_service, docker_service)
- Reuse a single Docker SDK client instead of reconnecting per customer
- Cache local-update-status results for 20s since the underlying data only
  changes after an image pull, not on every keystroke
- Parallelize /monitoring/customers container status lookups

Co-Authored-By: Claude Sonnet 5 <[email protected]>

app/services/docker_service.py

@@ -27,13 +27,24 @@ async def _run_cmd(cmd: list[str], timeout: int = 120) -> subprocess.CompletedPr
     )
 
 
+_client: Optional[docker.DockerClient] = None
+
+
 def _get_client() -> docker.DockerClient:
-    """Return a Docker client connected via the Unix socket.
+    """Return a shared Docker client connected via the Unix socket.
+
+    The client is created once and reused — creating a new client per call
+    (as `docker.from_env()` does) re-negotiates the API version and opens a
+    fresh connection every time, which is wasteful when called once per
+    customer in a loop.
 
     Returns:
         docker.DockerClient instance.
     """
-    return docker.from_env()
+    global _client
+    if _client is None:
+        _client = docker.from_env()
+    return _client
 
 
 async def compose_up(
@@ -212,6 +223,16 @@ def get_container_status(container_prefix: str) -> list[dict[str, Any]]:
     return results
 
 
+async def get_container_status_async(container_prefix: str) -> list[dict[str, Any]]:
+    """Thread-offloaded wrapper around get_container_status().
+
+    Use this when checking status for multiple customers so the Docker SDK
+    calls run in the thread pool instead of blocking the event loop.
+    """
+    loop = asyncio.get_event_loop()
+    return await loop.run_in_executor(None, get_container_status, container_prefix)
+
+
 def get_container_logs(container_name: str, tail: int = 200) -> str:
     """Retrieve recent logs from a container.