perf: eliminate source health N plus one queries

apps/sources/services/health.py

@@ -4,6 +4,7 @@ from collections.abc import Iterable
 from dataclasses import dataclass
 from datetime import datetime, timedelta
 
+from django.db.models import Prefetch
 from django.utils import timezone
 
 from apps.sources.models import Source, SourcePolicyReview, SourceRun
@@ -214,12 +215,17 @@ def _determine_health_action(runs: list[SourceRun]) -> tuple[str | None, str | N
 def collect_source_health(
     *, runs_to_consider: int = SOURCE_HEALTH_RUN_WINDOW
 ) -> list[SourceHealth]:
-    sources = Source.objects.order_by("name").all()
+    sources = Source.objects.prefetch_related(
+        Prefetch(
+            "runs",
+            queryset=SourceRun.objects.order_by("-started_at")[:runs_to_consider],
+            to_attr="health_runs",
+        )
+    ).order_by("name")
     rows: list[SourceHealth] = []
 
     for source in sources:
-        run_queryset = SourceRun.objects.filter(source=source).order_by("-started_at")
-        runs = list(run_queryset[:runs_to_consider])
+        runs = source.health_runs
         monitored_runs = len(runs)
         success_runs = [run for run in runs if run.status == SourceRun.Status.SUCCESS]