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/tests/test_docker_timeouts.py

@@ -0,0 +1,81 @@
+"""Slow Docker calls must fail with a message, not a bare 500.
+
+A volume or image prune that deletes a lot of data 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 long timeout, and any transport
+timeout that still happens is turned into a ``DockerError`` (-> 502 + detail).
+"""
+from __future__ import annotations
+
+import pytest
+import requests
+
+
+def test_safe_call_wraps_read_timeout():
+    from docker_client import DockerError, safe_call
+
+    def slow():
+        raise requests.exceptions.ReadTimeout("Read timed out. (read timeout=60)")
+
+    with pytest.raises(DockerError) as info:
+        safe_call(slow)
+    assert info.value.error == "docker_timeout"
+
+
+def test_safe_call_wraps_connection_error():
+    from docker_client import DockerError, safe_call
+
+    def gone():
+        raise requests.exceptions.ConnectionError("socket went away")
+
+    with pytest.raises(DockerError) as info:
+        safe_call(gone)
+    assert info.value.error == "docker_unavailable"
+
+
+def test_volume_prune_timeout_is_a_502_with_detail(as_admin, monkeypatch):
+    from services import volume_service
+
+    class Volumes:
+        def prune(self, filters=None):
+            raise requests.exceptions.ReadTimeout("Read timed out.")
+
+    class Client:
+        volumes = Volumes()
+
+    monkeypatch.setattr(volume_service, "get_long_client", lambda: Client())
+    r = as_admin.post("/api/volumes/prune")
+    assert r.status_code == 502
+    assert r.json()["error"] == "docker_timeout"
+
+
+@pytest.mark.parametrize(
+    "module, fn, args",
+    [
+        ("volume_service", "prune_volumes", ()),
+        ("image_service", "prune_images", (True,)),
+        ("network_service", "prune_networks", ()),
+    ],
+)
+def test_prunes_use_the_long_timeout_client(monkeypatch, module, fn, args):
+    import importlib
+
+    svc = importlib.import_module(f"services.{module}")
+    used = []
+
+    class Any_:
+        def prune(self, filters=None):
+            used.append("long")
+            return {}
+
+    class Client:
+        volumes = images = networks = Any_()
+
+    def short():
+        raise AssertionError("prune must not use the 60s client")
+
+    monkeypatch.setattr(svc, "get_client", short)
+    monkeypatch.setattr(svc, "get_long_client", lambda: Client())
+    getattr(svc, fn)(*args)
+    assert used == ["long"]