docs: update v1.5.0 docs - statistics API, PDF watermark, class teacher_id, sidebar order

docs/API.md

@@ -1066,6 +1066,47 @@ POST /api/config/test
 
 ---
 
+---
+
+## 数据统计 API
+
+### GET /api/statistics/students
+
+获取学员统计数据。
+
+**权限**: 登录用户
+
+**筛选参数**:
+| 参数 | 类型 | 说明 |
+|------|------|------|
+| mine | bool | 是否只看当前用户负责的班级（true/false） |
+| class_id | int | 筛选特定班级 |
+
+**响应示例**:
+```json
+{
+  "total_students": 15,
+  "total_problems": 42,
+  "level_distribution": {"启蒙": 3, "入门": 8, "进阶": 4, "熟练": 0, "精通": 0},
+  "severity_distribution": {"轻微": 12, "中等": 25, "严重": 5},
+  "class_student_count": [
+    {"class_id": 1, "class_name": "钢琴初级班", "level": "入门", "student_count": 8}
+  ],
+  "class_problem_count": [
+    {"class_id": 1, "class_name": "钢琴初级班", "problem_count": 20}
+  ],
+  "problem_level_matrix": {
+    "轻微": {"启蒙": 2, "入门": 6, "进阶": 3, "熟练": 1, "精通": 0},
+    "中等": {"启蒙": 1, "入门": 2, "进阶": 1, "熟练": 0, "精通": 0},
+    "严重": {"启蒙": 0, "入门": 0, "进阶": 0, "熟练": 0, "精通": 0}
+  },
+  "problem_name_distribution": {"手小": 5, "识谱慢": 8},
+  "classes": [{"id": 1, "name": "钢琴初级班"}]
+}
+```
+
+---
+
 ## 权限说明
 
 | 接口 | 管理员 | 普通用户 |