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>

apps/api/src/config.ts

@@ -7,4 +7,11 @@ export const config = {
   judgeServerUrl: process.env.JUDGE_SERVER_URL ?? "http://localhost:8081",
   judgeServerToken: process.env.JUDGE_SERVER_TOKEN ?? "oj2-dev-token",
   judgeConcurrency: Number(process.env.JUDGE_CONCURRENCY ?? 2),
+  avatarDirectory: process.env.AVATAR_DIRECTORY ?? "data/avatar",
+  avatarUriPrefix: process.env.AVATAR_URI_PREFIX ?? "/public/avatar",
+  aiBaseUrl: process.env.AI_BASE_URL ?? "https://api.deepseek.com",
+  aiKey: process.env.AI_KEY ?? "",
+  aiModel: process.env.AI_MODEL ?? "deepseek-v4-flash",
+  ruffPath: process.env.RUFF_PATH ?? "ruff",
+  clangFormatPath: process.env.CLANG_FORMAT_PATH ?? "clang-format",
 }