perf: aggregate batch status statistics in a single query

backend/wisegrade/api.py

@@ -468,12 +468,15 @@ def create_batch(data: BatchInput, db: Session = Depends(get_db)):
     return services.create_batch(data, db)
 
 
+def status_count_dict(counts: dict) -> dict:
+    return {"status_counts": counts, "needs_review": counts.get("needs_review", 0),
+            "unscorable": counts.get("unscorable", 0), "failed": counts.get("failed", 0)}
+
+
 def batch_status_counts(db, batch_id: int) -> dict:
     rows = db.execute(select(CriterionScore.status, func.count(CriterionScore.id))
                       .where(CriterionScore.batch_id == batch_id).group_by(CriterionScore.status)).all()
-    counts = {status: count for status, count in rows}
-    return {"status_counts": counts, "needs_review": counts.get("needs_review", 0),
-            "unscorable": counts.get("unscorable", 0), "failed": counts.get("failed", 0)}
+    return status_count_dict({status: count for status, count in rows})
 
 def batch_version_metadata(batch: ScoringBatch) -> dict:
     """Display the immutable rule/course versions, not their database IDs."""
@@ -485,12 +488,20 @@ def batch_version_metadata(batch: ScoringBatch) -> dict:
 
 @router.get("/scoring-batches")
 def list_batches(db: Session = Depends(get_db)):
+    batches = db.scalars(select(ScoringBatch).order_by(ScoringBatch.id.desc())).all()
+    counts_by_batch = {}
+    if batches:
+        rows = db.execute(select(CriterionScore.batch_id, CriterionScore.status, func.count(CriterionScore.id))
+                          .where(CriterionScore.batch_id.in_([batch.id for batch in batches]))
+                          .group_by(CriterionScore.batch_id, CriterionScore.status)).all()
+        for batch_id, status, count in rows:
+            counts_by_batch.setdefault(batch_id, {})[status] = count
     return [{**{key: getattr(batch, key) for key in ("id", "course_version_id", "rubric_version_id", "status", "progress_done", "progress_total", "error", "created_at", "finished_at")},
              "status_label": BATCH_STATUS_LABELS.get(batch.status, batch.status),
              "project_ids": batch.project_ids, "legacy_membership": batch.project_ids is None,
              **batch_version_metadata(batch),
-             **batch_status_counts(db, batch.id)}
-            for batch in db.scalars(select(ScoringBatch).order_by(ScoringBatch.id.desc()))]
+             **status_count_dict(counts_by_batch.get(batch.id, {}))}
+            for batch in batches]
 
 
 @router.get("/scoring-batches/{batch_id}/progress")