fix(阶段4评审收尾): 清掉三条 Minor，顺带一个真 bug

## M4 禁用账号会把学生卡在登录死循环里（唯一学生会撞上的）

`getSessionUser` 对禁用用户返回 null，于是落到 401 `login-required`，
而前端拦截器见到这个码就弹登录框 —— 一个上课上到一半被禁用的学生会陷入
「弹登录框 → 登进去 → 又被弹」，完全看不出发生了什么。

会话解析改成返回 `{ user } | { user: null, reason: "anonymous" | "disabled" }`，
禁用报 403 `account-disabled`（凭证有效、是账号不让用了，和 login 接口对禁用
账号的回法一致）。会话照删，禁用立即生效。前端补一支：清登录态 + 明确提示，
**不弹登录框**。

实测：会话中途 UPDATE is_disabled=true → 同一会话下一个请求
403 `account-disabled`「账号已被禁用，请联系老师」。

## M3 三个端点的守卫写在 handler 体内

submissions/statistics、submissions/:id/rejudge、flowcharts/statistics 的档位
本来就是对的，但写成 handler 里的 if，违背了「守卫要从注册行上看得出来」的约定，
下一个人加同类端点容易漏掉那个 if。改用 requireTeacher / requireSuperAdmin。

实测档位没变：普通学生三个都 403；教师统计接口 200、重判仍 403。

## M2 from-public 的错误码构成比赛存在性预言机

比赛不存在回 `not-found`、存在但不属于你回 `contest-not-found`，带一个已知
有效的 problemId 就能靠错误码枚举出哪些 contestId 真实存在。统一成
`contest-not-found`，和全仓其余跨租户路径一致。

实测：两种情况现在都是 404 contest-not-found。

## 顺带：比赛里的 SQL 题看不到示例数据

改 M2 时 tsc 报 `sqlDisplay` 声明了没用到 —— 查下去是真 bug：
`POST /contests/:id/problems` 把展示数据算出来了，却往库里写死 null
（公开题那两条路径都是对的）。于是比赛里的 SQL 题打开后没有示例数据表和期望结果。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/api/src/routes/admin/problem.ts

@@ -478,7 +478,9 @@ adminProblemRoutes.post("/contests/:contestId/problems", requireProblemPermissio
       acceptedNumber: 0,
       statisticInfo: {},
       isPublic: false,
-      sqlDisplay: null,
+      // 上面 generateSqlDisplay 已经把展示数据算好了，之前这里写死 null，
+      // 结果比赛里的 SQL 题打开后看不到示例数据表和期望结果（公开题那两条路径都是对的）
+      sqlDisplay,
     }).returning()
     await setTags(tx as unknown as typeof db, row!.id, parsed.data.tags)
     return row!
@@ -547,11 +549,14 @@ adminProblemRoutes.post("/contests/:contestId/problems/from-public", requireProb
   const [contest] = await db.select().from(schema.contest).where(eq(schema.contest.id, contestId)).limit(1)
   const [problem] = await db.select().from(schema.problem)
     .where(eq(schema.problem.id, parsed.data.problemId)).limit(1)
-  if (!contest || !problem) return failure(c, 404, "not-found", "Contest or Problem does not exist")
   const user = c.get("user")!
-  if (user.adminType !== "Super Admin" && contest.createdById !== user.id) {
-    return failure(c, 404, "contest-not-found", "Contest does not exist")
-  }
+  // 「比赛不存在」和「比赛存在但不是你的」必须回同一个码。分开报的话，带一个已知有效的
+  // problemId 就能靠错误码差异枚举出哪些 contestId 真实存在。全仓其余跨租户路径都是
+  // 统一码（contest 系列一律 contest-not-found），这里对齐。
+  const denyContest =
+    !contest || (user.adminType !== "Super Admin" && contest.createdById !== user.id)
+  if (denyContest) return failure(c, 404, "contest-not-found", "Contest does not exist")
+  if (!problem) return failure(c, 404, "problem-not-found", "Problem does not exist")
   // 源题必须是**公开题**，且要么已可见、要么是自己的。旧后端只按 id 取，不校验任何东西 ——
   // 于是能把别人比赛里的题（或别人尚未公开的草稿）拖进自己比赛，进而读到 answers。
   if (problem.contestId !== null) {

apps/api/src/routes/flowchart.ts

@@ -13,7 +13,7 @@ import {
 import { and, asc, count, desc, eq, ilike, isNull, sql } from "drizzle-orm"
 import { Hono } from "hono"
 
-import { requireAuth, type AppEnv } from "../auth/middleware"
+import { requireAuth, requireTeacher, type AppEnv } from "../auth/middleware"
 import { config } from "../config"
 import { db, schema } from "../db"
 import { failure, success } from "../http"
@@ -21,7 +21,6 @@ import { flowchartQueue } from "../queue"
 import { buildWordFrequencies } from "../services/word-frequency"
 import {
   isAdminRole,
-  isTeacherOrAbove,
   objectValue,
   queryInteger,
   rounded,
@@ -139,10 +138,7 @@ flowchartRoutes.get("/flowcharts", requireAuth, async (c) => {
 
 const FLOWCHART_COMPLETED = 2
 
-flowchartRoutes.get("/flowcharts/statistics", requireAuth, async (c) => {
-  if (!isTeacherOrAbove(c.get("user"))) {
-    return failure(c, 403, "permission-denied", "Teacher permission required")
-  }
+flowchartRoutes.get("/flowcharts/statistics", requireTeacher, async (c) => {
   const end = c.req.query("end")?.trim()
   if (!end) return failure(c, 400, "invalid-request", "end is required")
   const start = c.req.query("start")?.trim()

apps/api/src/routes/submission.ts

@@ -14,7 +14,12 @@ import {
 import { and, count, desc, eq, ilike, inArray, isNull, sql } from "drizzle-orm"
 import { Hono } from "hono"
 
-import { optionalAuth, requireAuth } from "../auth/middleware"
+import {
+  optionalAuth,
+  requireAuth,
+  requireSuperAdmin,
+  requireTeacher,
+} from "../auth/middleware"
 import type { AuthUser } from "../auth/session"
 import { db, schema } from "../db"
 import { failure, success } from "../http"
@@ -34,8 +39,6 @@ import { getBooleanOption } from "../services/options"
 import { consumeToken } from "../services/throttling"
 import {
   isAdminRole,
-  isSuperAdmin,
-  isTeacherOrAbove,
   queryInteger,
   rounded,
   stripClassPrefix,
@@ -218,10 +221,7 @@ async function matchedStudents(username: string) {
     )
 }
 
-submissionRoutes.get("/submissions/statistics", requireAuth, async (c) => {
-  if (!isTeacherOrAbove(c.get("user"))) {
-    return failure(c, 403, "permission-denied", "Teacher permission required")
-  }
+submissionRoutes.get("/submissions/statistics", requireTeacher, async (c) => {
   const range = statisticsRange(c)
   if (!range) return failure(c, 400, "invalid-request", "end is required")
 
@@ -334,10 +334,7 @@ submissionRoutes.get("/submissions/statistics", requireAuth, async (c) => {
   )
 })
 
-submissionRoutes.post("/submissions/:id/rejudge", requireAuth, async (c) => {
-  if (!isSuperAdmin(c.get("user"))) {
-    return failure(c, 403, "permission-denied", "Super admin permission required")
-  }
+submissionRoutes.post("/submissions/:id/rejudge", requireSuperAdmin, async (c) => {
   const [row] = await db
     .select({ id: schema.submission.id, problemId: schema.submission.problemId })
     .from(schema.submission)