- create crew_invitations table with status enum
- add CrewInvitation model with accept/reject flow
- add CrewInvitationsController for send/accept/reject
- add invitation error classes
- add invitation routes nested under crews
- add pending invitations endpoint for current user
- 38 passing specs for model and controller
- add transactional fixtures to rails_helper for test isolation
- restructure crew errors to CrewErrors module for Zeitwerk
- add rescue_from for CrewErrors::CrewError in api_controller
- add model specs for Crew and CrewMembership (34 examples)
- add controller specs for crews and memberships (28 examples)
- add crew-related specs to User model (22 examples)
- add factories for crews and crew_memberships
