Pin the UTC offset on the time a flow version carries

Every datetime the API emits for versions (flow summaries' updated_at,
runs' created/started/finished with flow_version) is already ISO 8601
with Z or +00:00. A test holds it and the API docs say so.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WgBfkm4rCmtd1T4vHWjEPx

backend/tests/api/routes/test_flows.py

@@ -62,6 +62,11 @@ def test_save_then_read_flow(
     assert body["definition"]["title"] == "Demo"
     assert {node["id"] for node in body["definition"]["nodes"]} == {"sensor", "logger"}
 
+    # The time beside a version carries its offset, so no client guesses a zone.
+    listed = client.get(f"{PREFIX}/", headers=superuser_token_headers).json()["data"]
+    summary = next(flow for flow in listed if flow["name"] == "demo")
+    assert datetime.fromisoformat(summary["updated_at"]).tzinfo is not None
+
 
 def test_name_mismatch_is_rejected(
     client: TestClient, superuser_token_headers: dict[str, str]