feat(阶段3): oj 侧端点铺开（基线提交，未经评审）

由外部 agent (Codex) 在本会话额度中断期间完成。原样提交作为基线，
后续修复单独成 commit，便于区分与回退。

覆盖 oj 侧 65 个端点，新增 9 组路由（account/achievement/ai/classroom/
content/contest/flowchart/problemset/site）与对应 Zod 契约。

已核验：tsc --noEmit 退出码 0；API 可启动；/api/problems 返回真实数据；
judge 与 flowchart worker 均 ready。
未核验：权限边界与数据泄露，评审进行中。

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

packages/contract/src/index.ts

@@ -1,3 +1,13 @@
 export * from "./auth"
+export * from "./account"
+export * from "./achievement"
+export * from "./ai"
+export * from "./classroom"
+export * from "./common"
+export * from "./content"
+export * from "./contest"
+export * from "./flowchart"
 export * from "./problem"
+export * from "./problemset"
+export * from "./site"
 export * from "./submission"