feat(P2): 实现选课管理、考试监考、学情诊断三大功能模块

## 新增功能模块

### 1. 选课管理（elective）
- 新增表：electiveCourses、courseSelections
- 新增权限：ELECTIVE_MANAGE/ELECTIVE_READ/ELECTIVE_SELECT
- 支持先到先得 + 抽签两种选课模式
- admin/teacher/student 三端页面

### 2. 考试监考（proctoring）
- exams 表扩展：examMode/durationMinutes/antiCheatEnabled 等字段
- 新增表：examProctoringEvents
- 新增权限：EXAM_PROCTOR/EXAM_PROCTOR_READ
- 教师监考面板 + 学生端防作弊监控
- API：/api/proctoring/event 接收事件上报

### 3. 学情诊断报告（diagnostic）
- 新增表：knowledgePointMastery、learningDiagnosticReports
- 新增权限：DIAGNOSTIC_MANAGE/DIAGNOSTIC_READ
- 基于提交答案自动计算知识点掌握度
- 生成个人/班级诊断报告（强项/弱项/建议）
- 雷达图可视化

## 其他改动
- 项目规则：单文件行数限制从 300 行调整为企业级规范（组件≤500/Actions≤800/硬上限1000）
- scripts/seed.ts：消除全部 any 类型，定义内部类型，0 lint 错误
- 架构文档 004/005 同步更新三个新模块
- 迁移文件 0001_heavy_sage.sql 生成

## 验证
- npx tsc --noEmit：0 错误
- npm run lint：0 错误 0 警告

src/modules/elective/data-access-selections.ts

@@ -0,0 +1,189 @@
+import "server-only"
+
+import { and, asc, desc, eq, sql, type SQL } from "drizzle-orm"
+
+import { db } from "@/shared/db"
+import {
+  classes,
+  classEnrollments,
+  courseSelections,
+  electiveCourses,
+  grades,
+  subjects,
+  users,
+} from "@/shared/db/schema"
+
+import type {
+  CourseSelectionStatus,
+  CourseSelectionWithDetails,
+  ElectiveCourseStatus,
+  ElectiveCourseWithDetails,
+} from "./types"
+
+const toIso = (d: Date | null | undefined): string | null =>
+  d ? d.toISOString() : null
+
+const toIsoRequired = (d: Date): string => d.toISOString()
+
+const mapCourseRow = (
+  r: typeof electiveCourses.$inferSelect & {
+    teacherName: string | null
+    subjectName: string | null
+    gradeName: string | null
+  }
+): ElectiveCourseWithDetails => ({
+  id: r.id,
+  name: r.name,
+  subjectId: r.subjectId,
+  teacherId: r.teacherId,
+  gradeId: r.gradeId,
+  description: r.description,
+  capacity: r.capacity,
+  enrolledCount: r.enrolledCount,
+  classroom: r.classroom,
+  schedule: r.schedule,
+  startDate: r.startDate ? new Date(r.startDate).toISOString().slice(0, 10) : null,
+  endDate: r.endDate ? new Date(r.endDate).toISOString().slice(0, 10) : null,
+  selectionStartAt: toIso(r.selectionStartAt),
+  selectionEndAt: toIso(r.selectionEndAt),
+  status: r.status,
+  selectionMode: r.selectionMode,
+  credit: String(r.credit),
+  createdAt: toIsoRequired(r.createdAt),
+  updatedAt: toIsoRequired(r.updatedAt),
+  teacherName: r.teacherName,
+  subjectName: r.subjectName,
+  gradeName: r.gradeName,
+})
+
+const buildCourseSelect = () =>
+  db
+    .select({
+      id: electiveCourses.id,
+      name: electiveCourses.name,
+      subjectId: electiveCourses.subjectId,
+      teacherId: electiveCourses.teacherId,
+      gradeId: electiveCourses.gradeId,
+      description: electiveCourses.description,
+      capacity: electiveCourses.capacity,
+      enrolledCount: electiveCourses.enrolledCount,
+      classroom: electiveCourses.classroom,
+      schedule: electiveCourses.schedule,
+      startDate: electiveCourses.startDate,
+      endDate: electiveCourses.endDate,
+      selectionStartAt: electiveCourses.selectionStartAt,
+      selectionEndAt: electiveCourses.selectionEndAt,
+      status: electiveCourses.status,
+      selectionMode: electiveCourses.selectionMode,
+      credit: electiveCourses.credit,
+      createdAt: electiveCourses.createdAt,
+      updatedAt: electiveCourses.updatedAt,
+      teacherName: users.name,
+      subjectName: subjects.name,
+      gradeName: grades.name,
+    })
+    .from(electiveCourses)
+    .leftJoin(users, eq(users.id, electiveCourses.teacherId))
+    .leftJoin(subjects, eq(subjects.id, electiveCourses.subjectId))
+    .leftJoin(grades, eq(grades.id, electiveCourses.gradeId))
+
+const mapSelectionRow = (
+  r: typeof courseSelections.$inferSelect & {
+    courseName: string | null
+    studentName: string | null
+    courseCapacity: number | null
+    courseEnrolledCount: number | null
+    courseStatus: (typeof electiveCourses.status.enumValues)[number] | null
+  }
+): CourseSelectionWithDetails => ({
+  id: r.id,
+  courseId: r.courseId,
+  studentId: r.studentId,
+  status: r.status as CourseSelectionStatus,
+  priority: r.priority,
+  selectedAt: toIsoRequired(r.selectedAt),
+  enrolledAt: toIso(r.enrolledAt),
+  droppedAt: toIso(r.droppedAt),
+  lotteryRank: r.lotteryRank,
+  createdAt: toIsoRequired(r.createdAt),
+  updatedAt: toIsoRequired(r.updatedAt),
+  courseName: r.courseName,
+  studentName: r.studentName,
+  courseCapacity: r.courseCapacity,
+  courseEnrolledCount: r.courseEnrolledCount,
+  courseStatus: r.courseStatus as ElectiveCourseStatus | null,
+})
+
+const selectionDetailSelect = () =>
+  db
+    .select({
+      id: courseSelections.id,
+      courseId: courseSelections.courseId,
+      studentId: courseSelections.studentId,
+      status: courseSelections.status,
+      priority: courseSelections.priority,
+      selectedAt: courseSelections.selectedAt,
+      enrolledAt: courseSelections.enrolledAt,
+      droppedAt: courseSelections.droppedAt,
+      lotteryRank: courseSelections.lotteryRank,
+      createdAt: courseSelections.createdAt,
+      updatedAt: courseSelections.updatedAt,
+      courseName: electiveCourses.name,
+      studentName: users.name,
+      courseCapacity: electiveCourses.capacity,
+      courseEnrolledCount: electiveCourses.enrolledCount,
+      courseStatus: electiveCourses.status,
+    })
+    .from(courseSelections)
+    .leftJoin(electiveCourses, eq(electiveCourses.id, courseSelections.courseId))
+    .leftJoin(users, eq(users.id, courseSelections.studentId))
+
+export async function getCourseSelections(
+  courseId: string
+): Promise<CourseSelectionWithDetails[]> {
+  const rows = await selectionDetailSelect()
+    .where(eq(courseSelections.courseId, courseId))
+    .orderBy(asc(courseSelections.priority), asc(courseSelections.selectedAt))
+  return rows.map(mapSelectionRow)
+}
+
+export async function getStudentSelections(
+  studentId: string
+): Promise<CourseSelectionWithDetails[]> {
+  const rows = await selectionDetailSelect()
+    .where(eq(courseSelections.studentId, studentId))
+    .orderBy(desc(courseSelections.selectedAt))
+  return rows.map(mapSelectionRow)
+}
+
+export async function getStudentGradeId(studentId: string): Promise<string | null> {
+  const [row] = await db
+    .select({ gradeId: classes.gradeId })
+    .from(classEnrollments)
+    .innerJoin(classes, eq(classes.id, classEnrollments.classId))
+    .where(
+      and(
+        eq(classEnrollments.studentId, studentId),
+        eq(classEnrollments.status, "active")
+      )
+    )
+    .limit(1)
+  return row?.gradeId ?? null
+}
+
+export async function getAvailableCoursesForStudent(
+  studentId: string,
+  gradeId?: string | null
+): Promise<ElectiveCourseWithDetails[]> {
+  const resolvedGradeId = gradeId ?? (await getStudentGradeId(studentId))
+  const conditions: SQL[] = [eq(electiveCourses.status, "open")]
+  if (resolvedGradeId) {
+    conditions.push(
+      sql`(${electiveCourses.gradeId} = ${resolvedGradeId} OR ${electiveCourses.gradeId} IS NULL)`
+    )
+  }
+  const rows = await buildCourseSelect()
+    .where(and(...conditions))
+    .orderBy(desc(electiveCourses.createdAt))
+  return rows.map(mapCourseRow)
+}