backend: add composite task listing indexes

backend/migrations/versions/b4338be78eec_add_composite_indexes_for_task_listing.py

@@ -0,0 +1,45 @@
+"""add composite indexes for task listing
+
+Revision ID: b4338be78eec
+Revises: f4d2b649e93a
+Create Date: 2026-02-12 07:54:27.450391
+
+"""
+from __future__ import annotations
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'b4338be78eec'
+down_revision = 'f4d2b649e93a'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Task list endpoints filter primarily by board_id, optionally by status
+    # and assigned_agent_id, and always order by created_at (desc in code).
+    # These composite btree indexes allow fast backward scans with LIMIT.
+    op.create_index(
+        "ix_tasks_board_id_created_at",
+        "tasks",
+        ["board_id", "created_at"],
+    )
+    op.create_index(
+        "ix_tasks_board_id_status_created_at",
+        "tasks",
+        ["board_id", "status", "created_at"],
+    )
+    op.create_index(
+        "ix_tasks_board_id_assigned_agent_id_created_at",
+        "tasks",
+        ["board_id", "assigned_agent_id", "created_at"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_tasks_board_id_assigned_agent_id_created_at", table_name="tasks")
+    op.drop_index("ix_tasks_board_id_status_created_at", table_name="tasks")
+    op.drop_index("ix_tasks_board_id_created_at", table_name="tasks")