feat: shift assignment workflow with claim, approve, reject, cancel, and bulk approve

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>

api/app/Models/Person.php

@@ -76,6 +76,11 @@ final class Person extends Model
         return $this->hasMany(ShiftAssignment::class);
     }
 
+    public function volunteerAvailabilities(): HasMany
+    {
+        return $this->hasMany(VolunteerAvailability::class);
+    }
+
     public function identityMatches(): HasMany
     {
         return $this->hasMany(PersonIdentityMatch::class);