feat(sp37): add batches.transaction_set_control_number column

Migration 0020 adds the column additively (nullable, no default) and
backfills from raw_result_json.envelope.transaction_set_control_number
for any existing batch rows that already carry it. Required for the
SP37 join-key update so 999 acks can resolve by ST02 (the source 837's
transaction set control number) instead of just ISA13.

backend/tests/test_db_migrate.py

@@ -126,14 +126,15 @@ def test_migration_latest_idempotent_on_fresh_db(tmp_path: Path) -> None:
     SP28 bumped it to 18 with the claim_acks join table.
     SP32 bumped it to 19 with rendering_provider_npi +
     service_provider_npi on claims and remittances.
+    SP37 bumped it to 20 with batch transaction_set_control_number.
     """
     engine = _fresh_engine(tmp_path)
     db_migrate.run(engine)
     v_after_first = _user_version(engine)
-    assert v_after_first == 19, f"expected head=19, got {v_after_first}"
+    assert v_after_first == 20, f"expected head=20, got {v_after_first}"
 
     db_migrate.run(engine)
-    assert _user_version(engine) == 19, "second run should not bump version"
+    assert _user_version(engine) == 20, "second run should not bump version"
 
 
 def test_drop_claims_unique_constraint_migration(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
@@ -159,7 +160,7 @@ def test_drop_claims_unique_constraint_migration(tmp_path: Path, monkeypatch: py
     engine = _fresh_engine(tmp_path)
 
     db_migrate.run(engine)
-    assert _user_version(engine) == 19, f"expected head=19, got {_user_version(engine)}"
+    assert _user_version(engine) == 20, f"expected head=20, got {_user_version(engine)}"
 
     # Two claims in one batch with the same patient_control_number
     # must be insertable. If 0015's table recreation re-introduced a