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/__init__.py

@@ -6,6 +6,7 @@ from app.routes.admin_routes import admin_bp
 from app.routes.auth_routes import auth_bp
 from app.routes.content_routes import content_bp
 from app.routes.spam_routes import spam_bp
+from app.routes.upload_routes import upload_bp
 from app.routes.user_routes import user_bp
 
 
@@ -22,6 +23,7 @@ def create_app() -> Flask:
     app.register_blueprint(spam_bp, url_prefix="/api/spam")
     app.register_blueprint(content_bp, url_prefix="/api/content")
     app.register_blueprint(admin_bp, url_prefix="/api/admin")
+    app.register_blueprint(upload_bp, url_prefix="/api/upload")
 
     @app.get("/api/health")
     def health_check():