feat(submissions): query bounded anonymous performance snapshots

backend/migrations/versions/0004_performance_watermark.py

@@ -0,0 +1,21 @@
+"""Indexed group watermark for bounded aggregate cache invalidation."""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0004_performance_watermark"
+down_revision = "0003_submission_performance"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_index(
+        "ix_performance_group_watermark",
+        "evaluation_performance",
+        ["comparison_group", sa.text("finished_at DESC"), sa.text("evaluation_id DESC")],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_performance_group_watermark", table_name="evaluation_performance")