feat: 申诉常见理由快捷选择 + 证据截图上传

- 后端: 新增 appeal_reason_type, appeal_evidence_urls 字段
- 后端: 新建 upload_routes.py 图片上传接口
- 前端: history 页面添加快捷理由选择器 + 截图上传
- 前端: admin-review 页面展示证据图片 + 点击预览
- 新增 SQL 更新脚本 update_appeal_fields.sql

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/config.py

@@ -35,3 +35,4 @@ class Config:
 
     SPAM_DATASET_PATH = str(BASE_DIR / "seed" / "spam_samples_seed.json")
     NB_MODEL_PATH = str(BASE_DIR / "models" / "spam_nb_model.joblib")
+    UPLOAD_FOLDER = str(BASE_DIR / "uploads")