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/models.py

@@ -141,7 +141,9 @@ class ContentPost(db.Model):
     manual_review_at = db.Column(db.DateTime, nullable=True)
 
     appeal_status = db.Column(db.String(16), nullable=False, default="none")  # none | pending | approved | rejected
+    appeal_reason_type = db.Column(db.String(32), default="")  # 快捷理由类型
     appeal_reason = db.Column(db.String(255), default="")
+    appeal_evidence_urls = db.Column(db.JSON, default=list)  # 证据图片 URL 列表
     appeal_admin_note = db.Column(db.String(255), default="")
     appeal_submitted_at = db.Column(db.DateTime, nullable=True)
     appeal_processed_at = db.Column(db.DateTime, nullable=True)
@@ -170,7 +172,9 @@ class ContentPost(db.Model):
             "manual_review_note": self.manual_review_note,
             "manual_review_at": self.manual_review_at.isoformat() if self.manual_review_at else None,
             "appeal_status": self.appeal_status,
+            "appeal_reason_type": self.appeal_reason_type,
             "appeal_reason": self.appeal_reason,
+            "appeal_evidence_urls": self.appeal_evidence_urls or [],
             "appeal_admin_note": self.appeal_admin_note,
             "appeal_submitted_at": self.appeal_submitted_at.isoformat() if self.appeal_submitted_at else None,
             "appeal_processed_at": self.appeal_processed_at.isoformat() if self.appeal_processed_at else None,