Add cancelled_by, cancellation_source (organiser|volunteer|system), and
cancelled_at columns to shift_assignments. Cancel flow now records who
cancelled and why. Assign flow reactivates existing cancelled/rejected
records instead of creating duplicates, preventing UNIQUE constraint
violations. Assignable-persons endpoint returns previous_assignment data
for contextual UI indicators. Frontend shows cancellation source labels,
previous assignment history in assign dialog, and "Opnieuw toewijzen"
buttons with volunteer-cancelled confirmation dialogs.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Implements the complete ShiftAssignment lifecycle:
- ShiftAssignmentStatus enum with allowed transitions
- ShiftAssignmentService with claim/assign/approve/reject/cancel/bulkApprove
- ShiftAssignmentController with event-scoped endpoints
- ShiftAssignmentPolicy (organizer + volunteer self-cancel)
- VolunteerAvailability model, controller, and sync endpoint
- Refactored ShiftController to delegate to service layer
- 31 workflow tests covering all paths and multi-tenancy
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
