feat: 多班级版 v2.0 - Go后端重写 + 43轮代码审查

- 后端从 Python FastAPI 重写为 Go Gin（端口 56789）
- 多班级完全隔离
- 超级管理员独立登录
- 课代表作业管理、排行榜分项排行
- 角色加减分上下限可配置
- 家长改密功能（可开关）
- 周度/月度重置功能
- MySQL 5.7 兼容
- 43轮代码审查+全部修复
- Apache 2.0 许可证

frontend/assets/uploads/sample_import.json

@@ -1,49 +1,23 @@
 {
-  "_comment1": "================================================",
-  "_comment2": "班级操行分管理系统 - 学生批量导入模板",
-  "_comment3": "开发者: Canglan | 版权: Sea Network Technology Studio",
-  "_comment4": "================================================",
-  "_comment5": "字段说明：",
-  "_comment6": "  student_no      - 必填，学生学号，唯一标识",
-  "_comment7": "  name           - 必填，学生姓名",
-  "_comment8": "  parent_phone   - 可选，家长手机号（11位手机号）",
-  "_comment9": "  dormitory_number - 可选，宿舍号（支持字母数字组合，如 301-A）",
-  "_comment10": "  password       - 可选，初始密码，不填则默认 123456",
-  "_comment11": "================================================",
-  "_comment12": "导入规则：",
-  "_comment13": "  1. 学生操行分初始值 = 60分",
-  "_comment14": "  2. 学生账号 = 学号，密码 = 指定的password或123456",
-  "_comment15": "  3. 家长账号 = 手机号（若parent_phone有值），密码 = 指定的password或123456",
-  "_comment16": "  4. 家长姓名默认显示为 '学生姓名家长'",
-  "_comment17": "================================================",
-  "students": [
-    {
-      "student_no": "20240001",
-      "name": "张三",
-      "parent_phone": "13800138001",
-      "dormitory_number": "301-A",
-      "password": "123456"
-    },
-    {
-      "student_no": "20240002",
-      "name": "李四",
-      "parent_phone": "13800138002",
-      "dormitory_number": "205",
-      "password": "123456"
-    },
-    {
-      "student_no": "20240003",
-      "name": "王五",
-      "parent_phone": "",
-      "dormitory_number": "",
-      "password": ""
-    },
-    {
-      "student_no": "20240004",
-      "name": "赵六",
-      "parent_phone": "13800138004",
-      "dormitory_number": "102-B",
-      "password": ""
-    }
-  ]
-}
+    "students": [
+        {
+            "student_no": "2025001",
+            "name": "张三",
+            "parent_account": "13800138001",
+            "dormitory_number": "A301",
+            "password": "123456"
+        },
+        {
+            "student_no": "2025002",
+            "name": "李四",
+            "parent_account": "13800138002",
+            "dormitory_number": "A302"
+        },
+        {
+            "student_no": "2025003",
+            "name": "王五",
+            "parent_account": "",
+            "dormitory_number": "B101"
+        }
+    ]
+}