feat(fc2c-i): migration 0004 enables tsm_system_rows

alembic/versions/0004_fc2c_i_tsm_system_rows.py

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+"""fc2c-i: enable tsm_system_rows for scalable random sampling
+
+Revision ID: 0004
+Revises: 0003
+Create Date: 2026-05-15
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0004"
+down_revision: Union[str, None] = "0003"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute("CREATE EXTENSION IF NOT EXISTS tsm_system_rows")
+
+
+def downgrade() -> None:
+    op.execute("DROP EXTENSION IF EXISTS tsm_system_rows")

tests/test_migration_0004.py

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+"""Integration: the tsm_system_rows extension is installed by migration 0004.
+
+Needs a real Postgres (CI does not provision one), so integration-marked.
+"""
+
+import pytest
+from sqlalchemy import text
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.mark.asyncio
+async def test_tsm_system_rows_extension_present(db):
+    row = (
+        await db.execute(
+            text("SELECT 1 FROM pg_extension WHERE extname = 'tsm_system_rows'")
+        )
+    ).first()
+    assert row is not None
+
+
+@pytest.mark.asyncio
+async def test_system_rows_sampling_is_usable(db):
+    # Should parse and execute even on an empty table.
+    await db.execute(
+        text("SELECT * FROM image_record TABLESAMPLE SYSTEM_ROWS(1)")
+    )