- 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.
Adds migration 0015_drop_claims_unique_constraint.sql that recreates the
claims table without the inline UNIQUE(batch_id, patient_control_number)
constraint, plus tests proving the migration runs cleanly and the
constraint is not re-introduced.
Tyler