feat: 添加密码重置功能

- 后端: 新增 POST /auth/reset-password 端点（邮箱+验证码+新密码）
- 后端: 新增 ResetPasswordRequest schema
- 前端: 新增 /forgot-password 页面（分步骤：输入邮箱→验证码+新密码→完成）
- 前端: 登录页添加"忘记密码？"链接

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

backend/app/schemas/auth.py

@@ -81,6 +81,22 @@ class BindEmailRequest(BaseModel):
     password: str = Field(..., min_length=6, max_length=128)
 
 
+class ResetPasswordRequest(BaseModel):
+    """重置密码请求（通过邮箱验证码）"""
+    email: EmailStr
+    email_code: str = Field(..., min_length=4, max_length=8)
+    new_password: str = Field(..., min_length=6, max_length=128)
+
+    class Config:
+        json_schema_extra = {
+            "example": {
+                "email": "user@example.com",
+                "email_code": "123456",
+                "new_password": "newpassword123"
+            }
+        }
+
+
 class ChangePasswordRequest(BaseModel):
     """修改密码请求"""
     old_password: str