feat:YouTube Shorts AI 자동 생성 시스템 구현 (Veo 3.1 + Gemini)

- GeminiScriptService: 트렌딩 제목/시나리오 생성
- VeoVideoService: Veo 3.1 영상 클립 생성
- TtsService: Google TTS 나레이션 생성
- BgmService: 분위기별 BGM 선택
- VideoAssemblyService: FFmpeg 영상 합성
- VideoGenerationJob: 백그라운드 처리
- Veo3Controller: API 엔드포인트
- React 프론트엔드 (5단계 위저드)
- GoogleCloudService.getAccessToken() public 변경

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

app/Models/VideoGeneration.php

@@ -0,0 +1,85 @@
+<?php
+
+namespace App\Models;
+
+use App\Models\Scopes\TenantScope;
+use Illuminate\Database\Eloquent\Model;
+
+class VideoGeneration extends Model
+{
+    protected $fillable = [
+        'tenant_id',
+        'user_id',
+        'keyword',
+        'title',
+        'scenario',
+        'status',
+        'progress',
+        'current_step',
+        'clips_data',
+        'output_path',
+        'cost_usd',
+        'error_message',
+    ];
+
+    protected $casts = [
+        'scenario' => 'array',
+        'clips_data' => 'array',
+        'progress' => 'integer',
+        'cost_usd' => 'decimal:4',
+    ];
+
+    protected static function booted(): void
+    {
+        static::addGlobalScope(new TenantScope);
+    }
+
+    public function user()
+    {
+        return $this->belongsTo(\App\Models\User::class);
+    }
+
+    /**
+     * 상태 상수
+     */
+    const STATUS_PENDING = 'pending';
+
+    const STATUS_TITLES_GENERATED = 'titles_generated';
+
+    const STATUS_SCENARIO_READY = 'scenario_ready';
+
+    const STATUS_GENERATING_TTS = 'generating_tts';
+
+    const STATUS_GENERATING_CLIPS = 'generating_clips';
+
+    const STATUS_GENERATING_BGM = 'generating_bgm';
+
+    const STATUS_ASSEMBLING = 'assembling';
+
+    const STATUS_COMPLETED = 'completed';
+
+    const STATUS_FAILED = 'failed';
+
+    /**
+     * 진행 상태 업데이트 헬퍼
+     */
+    public function updateProgress(string $status, int $progress, string $step): void
+    {
+        $this->update([
+            'status' => $status,
+            'progress' => $progress,
+            'current_step' => $step,
+        ]);
+    }
+
+    /**
+     * 실패 처리
+     */
+    public function markFailed(string $errorMessage): void
+    {
+        $this->update([
+            'status' => self::STATUS_FAILED,
+            'error_message' => $errorMessage,
+        ]);
+    }
+}