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/VolunteerAvailability.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Concerns\HasUlids;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+final class VolunteerAvailability extends Model
+{
+    use HasFactory;
+    use HasUlids;
+
+    public $timestamps = false;
+
+    protected $table = 'volunteer_availabilities';
+
+    protected $fillable = [
+        'person_id',
+        'time_slot_id',
+        'preference_level',
+        'submitted_at',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'preference_level' => 'integer',
+            'submitted_at' => 'datetime',
+        ];
+    }
+
+    public function person(): BelongsTo
+    {
+        return $this->belongsTo(Person::class);
+    }
+
+    public function timeSlot(): BelongsTo
+    {
+        return $this->belongsTo(TimeSlot::class);
+    }
+}