New /trips router with full CRUD, attendee management, and nested
events. Events validate date is within parent trip range and inherit
trip location when not explicitly set. Leaving a trip cascades
removal from all its events.
New TripCommands cog with /trip create, list, view, delete, join,
leave and /trip event add, remove, join, leave. Event autocomplete
is scoped to the selected trip. Enforces must-be-on-trip rule for
event joins with a clear error message.
Logan