Replace the create_fee_type/create_cycle helpers duplicated across 18/8
membership-fee test files with a single shared definition in Mv.Fixtures,
reconciling the divergent local signatures (including the reversed
argument order) into one superset so behavior is unchanged.
- Remove Process.sleep calls from integration tests (tests run synchronously in SQL sandbox)
- Improve error handling: membership_fee_type_not_found now returns changeset error instead of just logging
- Clarify partial_failure documentation: successful_cycles are not persisted on rollback
- Update documentation: joined_at → join_date, left_at → exit_date
- Document PostgreSQL advisory locks per member (not whole table lock)
- Document gap handling: explicitly deleted cycles are not recreated
- Add create_member_with_cycles helper that uses fixed 'today' date
- Update tests to use explicit 'today:' option instead of Date.utc_today()
- Prevents test failures when current date changes (e.g., in 2026+)
- Tests now explicitly delete and regenerate cycles with fixed dates
- Ensures consistent test behavior regardless of execution date
