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/Shift.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace App\Models;
 
+use App\Enums\ShiftAssignmentStatus;
 use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Casts\Attribute;
 use Illuminate\Database\Eloquent\Concerns\HasUlids;
@@ -99,7 +100,7 @@ final class Shift extends Model
                 return (int) $this->attributes['filled_slots'];
             }
 
-            return $this->shiftAssignments()->where('status', 'approved')->count();
+            return $this->shiftAssignments()->where('status', ShiftAssignmentStatus::APPROVED)->count();
         });
     }