fix(sp22): correct test fixtures in SP22 migration tests

- test_migration_latest_idempotent_on_fresh_db now uses the pytest
  tmp_path fixture instead of a literal /tmp path that did not exist,
  so sqlite can create the test DB.
- test_drop_claims_unique_constraint_migration now inserts the NOT
  NULL batches.kind / input_filename / parsed_at columns when seeding
  the parent row for the two same-PCN claims.

backend/tests/test_db_migrate.py

@@ -115,12 +115,12 @@ def test_run_ignores_non_sql_files(
     assert _user_version(engine) == 1
 
 
-def test_migration_latest_idempotent_on_fresh_db() -> None:
+def test_migration_latest_idempotent_on_fresh_db(tmp_path: Path) -> None:
     """All migrations up to the current head run cleanly on a fresh DB,
     and a second run is a no-op (no version bump). SP22 bumped the
     expected head from 14 to 15 with the new UNIQUE-drop migration.
     """
-    engine = _fresh_engine(Path("/tmp/sp22_fresh_db_latest_test.db"))
+    engine = _fresh_engine(tmp_path)
     db_migrate.run(engine)
     v_after_first = _user_version(engine)
     assert v_after_first == 15, f"expected head=15, got {v_after_first}"
@@ -161,7 +161,8 @@ def test_drop_claims_unique_constraint_migration(tmp_path: Path, monkeypatch: py
     # claims to batches still works after the recreation.)
     with engine.begin() as conn:
         conn.exec_driver_sql(
-            "INSERT INTO batches (id) VALUES ('B1')"
+            "INSERT INTO batches (id, kind, input_filename, parsed_at) "
+            "VALUES ('B1', '837p', 'test.txt', '2026-01-01 00:00:00')"
         )
         conn.exec_driver_sql(
             "INSERT INTO claims (id, batch_id, patient_control_number, charge_amount) "