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/database/factories/ShiftAssignmentFactory.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace Database\Factories;
 
+use App\Enums\ShiftAssignmentStatus;
 use App\Models\Person;
 use App\Models\Shift;
 use App\Models\ShiftAssignment;
@@ -20,15 +21,16 @@ final class ShiftAssignmentFactory extends Factory
             'shift_id' => Shift::factory(),
             'person_id' => Person::factory(),
             'time_slot_id' => TimeSlot::factory(),
-            'status' => 'pending_approval',
+            'status' => ShiftAssignmentStatus::PENDING_APPROVAL,
             'auto_approved' => false,
+            'assigned_at' => now(),
         ];
     }
 
     public function approved(): static
     {
         return $this->state(fn () => [
-            'status' => 'approved',
+            'status' => ShiftAssignmentStatus::APPROVED,
             'approved_at' => now(),
         ]);
     }
@@ -36,7 +38,7 @@ final class ShiftAssignmentFactory extends Factory
     public function autoApproved(): static
     {
         return $this->state(fn () => [
-            'status' => 'approved',
+            'status' => ShiftAssignmentStatus::APPROVED,
             'auto_approved' => true,
             'approved_at' => now(),
         ]);