feat:AI 토큰 사용량 추적 기능 추가

- ai_token_usages 테이블 마이그레이션 생성
- AiTokenUsage 모델 생성
- AiReportService에 usageMetadata 추출 및 저장 로직 추가

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/Models/Tenants/AiTokenUsage.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace App\Models\Tenants;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class AiTokenUsage extends Model
+{
+    protected $table = 'ai_token_usages';
+
+    protected $fillable = [
+        'tenant_id',
+        'model',
+        'menu_name',
+        'prompt_tokens',
+        'completion_tokens',
+        'total_tokens',
+        'cost_usd',
+        'cost_krw',
+        'request_id',
+        'created_by',
+    ];
+
+    protected $casts = [
+        'prompt_tokens' => 'integer',
+        'completion_tokens' => 'integer',
+        'total_tokens' => 'integer',
+        'cost_usd' => 'decimal:6',
+        'cost_krw' => 'decimal:2',
+    ];
+
+    public function creator(): BelongsTo
+    {
+        return $this->belongsTo(\App\Models\Members\User::class, 'created_by');
+    }
+}