refactor(grades,diagnostic): 完成成绩和学情诊断模块审计 P1+P2 改进项

P1-1: 抽取 stats-service.ts，将 8 个统计计算纯函数从 data-access 层分离
P1-5: 创建 WidgetBoundary 组件 + 补齐 teacher 路由 loading.tsx/error.tsx (14 文件)
P1-6: 同步架构图文档 004/005，新增 stats-service 与 widget-boundary 节点
P2-1: 补充 a11y ARIA 属性（5 图表 role=img + aria-label，2 表格 caption，3 列表 role=list，3 按钮 aria-label）
P2-3: 修复班级报告 studentId 字段语义错误（schema 改为可空 + 迁移 + 代码适配）
P2-4: 修复 grade_managed scope 返回空数据（改为子查询 classes 表按 gradeId 过滤）
P2-5: 新增 /parent/diagnostic/ 页面（多子女学情诊断聚合 + loading + error）
P2-6: 统一 SearchParams 工具（student/grades 和 management/grade/insights 改用 @/shared/lib/search-params）

src/modules/grades/data-access-ranking.ts

@@ -9,8 +9,8 @@ import { getStudentActiveClassId } from "@/modules/classes/data-access"
 import { getUserNamesByIds } from "@/modules/users/data-access"
 
 import { normalize, toNumber } from "./lib/grade-utils"
+import { buildRankingTrendPoints, type RankingTrendEntry } from "./stats-service"
 import type {
-  RankingTrendPoint,
   RankingTrendResult,
 } from "./types"
 
@@ -56,13 +56,7 @@ export const getRankingTrend = cache(
       .where(and(...conditions))
       .orderBy(asc(gradeRecords.createdAt))
 
-    const byTitle = new Map<
-      string,
-      {
-        date: Date
-        entries: Array<{ studentId: string; normalized: number }>
-      }
-    >()
+    const byTitle = new Map<string, RankingTrendEntry>()
 
     for (const r of rows) {
       const entry = byTitle.get(r.title) ?? { date: r.createdAt, entries: [] }
@@ -73,33 +67,7 @@ export const getRankingTrend = cache(
       byTitle.set(r.title, entry)
     }
 
-    const points: RankingTrendPoint[] = []
-    for (const [title, entry] of byTitle.entries()) {
-      if (entry.entries.length === 0) continue
-      const sorted = [...entry.entries].sort((a, b) => b.normalized - a.normalized)
-      // Single traversal: find rank and student entry together
-      let rank = 0
-      let studentEntry: { studentId: string; normalized: number } | null = null
-      for (let i = 0; i < sorted.length; i += 1) {
-        const e = sorted[i]
-        if (e.studentId === studentId) {
-          rank = i + 1
-          studentEntry = e
-          break
-        }
-      }
-      if (rank <= 0 || !studentEntry) continue
-
-      points.push({
-        title,
-        date: entry.date.toISOString(),
-        score: studentEntry.normalized,
-        rank,
-        totalStudents: sorted.length,
-      })
-    }
-
-    points.sort((a, b) => new Date(a.date).getTime() - new Date(b.date).getTime())
+    const points = buildRankingTrendPoints(byTitle, studentId)
 
     return {
       studentId,