- 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
- crews table with name, gamertag, granblue_crew_id, description
- crew_memberships with role enum (member/vice_captain/captain)
- partial unique index ensures one active crew per user
- updated User model with crew associations and helper methods
