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/flowchart.ts

@@ -26,7 +26,7 @@ import { Hono } from "hono"
 import { requireAuth, requireTeacher, type AppEnv } from "../auth/middleware"
 import { config } from "../config"
 import { db, schema } from "../db"
-import { failure, success } from "../http"
+import { failure, readJson, success } from "../http"
 import { flowchartQueue } from "../queue"
 import { getBooleanOption } from "../services/options"
 import { consumeToken } from "../services/throttling"
@@ -85,9 +85,7 @@ function flowchartData(
 }
 
 flowchartRoutes.post("/flowcharts", requireAuth, async (c) => {
-  const parsed = createFlowchartRequestSchema.safeParse(
-    await c.req.json().catch(() => null),
-  )
+  const parsed = await readJson(c, createFlowchartRequestSchema)
   if (
     !parsed.success ||
     JSON.stringify(parsed.data?.flowchartData ?? {}).length > 500 * 1024