perf: eliminate source health N plus one queries

tests/integration/test_source_health.py

@@ -111,6 +111,25 @@ def test_collect_source_health_exposes_parser_and_counts(source):
     assert row.updated_count == 1
 
 
+@pytest.mark.django_db
+def test_collect_source_health_prefetches_runs_for_all_sources(source, django_assert_num_queries):
+    second_source = Source.objects.create(
+        name="Second jobs",
+        source_type=Source.Type.EMPLOYER,
+        base_url="https://second.example.org/jobs/",
+        domain="second.example.org",
+        status=Source.Status.ACTIVE,
+        policy=Source.Policy.ALLOW,
+    )
+    _create_run(source, status=SourceRun.Status.SUCCESS)
+    _create_run(second_source, status=SourceRun.Status.SUCCESS)
+
+    with django_assert_num_queries(2):
+        rows = collect_source_health()
+
+    assert [row.source_name for row in rows] == ["Example jobs", "Second jobs"]
+
+
 @pytest.mark.django_db
 def test_recover_source_health_starts_canary_once(source, monkeypatch):
     source.status = Source.Status.QUARANTINED