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/Enums/ShiftAssignmentStatus.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Enums;
+
+enum ShiftAssignmentStatus: string
+{
+    case PENDING_APPROVAL = 'pending_approval';
+    case APPROVED = 'approved';
+    case REJECTED = 'rejected';
+    case CANCELLED = 'cancelled';
+    case COMPLETED = 'completed';
+
+    /** @return list<self> */
+    public function allowedTransitions(): array
+    {
+        return match ($this) {
+            self::PENDING_APPROVAL => [self::APPROVED, self::REJECTED, self::CANCELLED],
+            self::APPROVED => [self::CANCELLED, self::COMPLETED],
+            self::REJECTED, self::CANCELLED, self::COMPLETED => [],
+        };
+    }
+
+    public function canTransitionTo(self $target): bool
+    {
+        return in_array($target, $this->allowedTransitions(), true);
+    }
+
+    public function isTerminal(): bool
+    {
+        return $this->allowedTransitions() === [];
+    }
+}