Stop long prunes failing with a bare 500 (0.60.1)

A prune that deletes a lot of data (image prune -a in particular) easily
runs past docker-py's 60s read timeout. requests' ReadTimeout is not a
DockerException, so it slipped past safe_call and the UI only showed
"Request failed with status code 500".

Prunes now use a client with a 540s timeout (below nginx's 600s
proxy_read_timeout), and safe_call turns transport timeouts and
connection errors into DockerError, so they surface as a 502 with a
message.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/services/volume_service.py

@@ -7,7 +7,7 @@ from typing import Literal, Optional
 
 import yaml
 
-from docker_client import get_client, safe_call
+from docker_client import get_client, get_long_client, safe_call
 
 COMPOSE_PROJECT_LABEL = "com.docker.compose.project"
 
@@ -86,7 +86,7 @@ def remove_volume(name: str, force: bool = False) -> None:
 
 
 def prune_volumes() -> dict:
-    client = get_client()
+    client = get_long_client()
     return safe_call(client.volumes.prune)