feat: 添加邮件功能第五阶段 - 用户邮箱绑定

1. 添加邮箱绑定验证邮件模板 (templates/email/bind_email.html)
2. 在email_service.py中添加:
   - send_bind_email_verification() 发送绑定验证邮件
   - verify_bind_email_token() 验证绑定Token
3. 在database.py中添加:
   - update_user_email() 更新用户邮箱
4. 在app.py中添加API:
   - GET /api/user/email - 获取用户邮箱信息
   - POST /api/user/bind-email - 发送绑定验证邮件
   - GET /api/verify-bind-email/<token> - 验证绑定Token
   - POST /api/user/unbind-email - 解绑邮箱
5. 更新templates/index.html:
   - 将"修改密码"弹窗改为"个人设置"
   - 添加邮箱绑定/解绑功能UI
   - 显示邮箱状态（未绑定/待验证/已验证）

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

database.py

@@ -830,6 +830,19 @@ def get_user_by_email(email):
         return dict(user) if user else None
 
 
+def update_user_email(user_id, email, verified=False):
+    """更新用户邮箱"""
+    with db_pool.get_db() as conn:
+        cursor = conn.cursor()
+        cursor.execute('''
+            UPDATE users
+            SET email = ?, email_verified = ?
+            WHERE id = ?
+        ''', (email, int(verified), user_id))
+        conn.commit()
+        return cursor.rowcount > 0
+
+
 def get_all_users():
     """获取所有用户"""
     with db_pool.get_db() as conn: