refactor(api): 请求体解析收成 readJson / parseBody

54 处 handler 都手写同一段：
  const parsed = X.safeParse(await c.req.json().catch(() => null))
  if (!parsed.success) return failure(c, 400, "invalid-request", ...)
漏写 .catch 就是一个畸形请求打出 500。

- http.ts 加 readJson（读 + 校验）和 parseBody（再加上 400）。parseBody 不给
  message 时回 zod 的第一条 issue —— 后台表单原来就是这么写的，fallback 文案
  「Invalid payload」/「参数错误」只在 issues 为空时才用得上，实际走不到
- 51 处换成 parseBody；另 3 处失败时带别的错误码或和路径参数一起判，换成 readJson
- 错误码、状态码、文案、判断顺序都不变

实跑核过：非 JSON 请求体回原来的固定文案，公告标题为空回 zod issue。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/src/routes/classroom.ts

@@ -11,7 +11,7 @@ import { Hono } from "hono"
 
 import { requireAuth, type AppEnv } from "../auth/middleware"
 import { db, schema } from "../db"
-import { failure, success } from "../http"
+import { failure, parseBody, success } from "../http"
 import { JudgeStatus } from "../judge/status"
 import { queryInteger, rounded } from "./helpers"
 
@@ -165,11 +165,12 @@ classroomRoutes.get("/me/class-rank", requireAuth, async (c) => {
 })
 
 classroomRoutes.post("/classes/comparison", async (c) => {
-  const parsed = classComparisonRequestSchema.safeParse(
-    await c.req.json().catch(() => null),
+  const parsed = await parseBody(
+    c,
+    classComparisonRequestSchema,
+    "At least one class is required",
   )
-  if (!parsed.success)
-    return failure(c, 400, "invalid-request", "At least one class is required")
+  if (!parsed.success) return parsed.response
   const users = await loadClassUsers(parsed.data.classNames)
   const allAc = users.map((user) => user.acceptedNumber)
   const globalQ1 = quantile(allAc, 0.25)