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/Http/Resources/Api/V1/ShiftAssignmentResource.php

@@ -15,10 +15,24 @@ final class ShiftAssignmentResource extends JsonResource
             'id' => $this->id,
             'shift_id' => $this->shift_id,
             'person_id' => $this->person_id,
-            'status' => $this->status,
+            'time_slot_id' => $this->time_slot_id,
+            'status' => $this->status->value,
             'auto_approved' => $this->auto_approved,
+            'assigned_by' => $this->assigned_by,
             'assigned_at' => $this->assigned_at?->toIso8601String(),
+            'approved_by' => $this->approved_by,
             'approved_at' => $this->approved_at?->toIso8601String(),
+            'rejection_reason' => $this->rejection_reason,
+            'hours_expected' => $this->hours_expected,
+            'hours_completed' => $this->hours_completed,
+            'checked_in_at' => $this->checked_in_at?->toIso8601String(),
+            'checked_out_at' => $this->checked_out_at?->toIso8601String(),
+            'is_cancellable' => $this->isCancellable(),
+            'is_approvable' => $this->isApprovable(),
+            'created_at' => $this->created_at?->toIso8601String(),
+
+            'person' => new PersonResource($this->whenLoaded('person')),
+            'shift' => new ShiftResource($this->whenLoaded('shift')),
         ];
     }
 }