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/contest.ts

@@ -0,0 +1,42 @@
+import { z } from "zod"
+
+import { paginatedSchema, sampleUserSchema } from "./common"
+import { problemDetailSchema, problemListItemSchema } from "./problem"
+
+export const contestStatusSchema = z.enum(["-1", "0", "1"])
+
+export const contestSchema = z.object({
+  id: z.number().int(),
+  title: z.string(),
+  description: z.string(),
+  tag: z.string(),
+  startTime: z.string(),
+  endTime: z.string(),
+  createTime: z.string(),
+  lastUpdateTime: z.string(),
+  createdBy: sampleUserSchema,
+  status: contestStatusSchema,
+  contestType: z.enum(["Public", "Password Protected"]),
+  now: z.string().optional(),
+})
+
+export const contestListSchema = paginatedSchema(contestSchema)
+
+export const contestPasswordRequestSchema = z.object({
+  password: z.string().min(1).max(128),
+})
+
+export const contestAccessSchema = z.object({ access: z.boolean() })
+export const contestProblemsSchema = z.array(z.union([problemListItemSchema, problemDetailSchema]))
+
+export const contestRankItemSchema = z.object({
+  id: z.number().int(),
+  user: sampleUserSchema,
+  submissionNumber: z.number().int(),
+  acceptedNumber: z.number().int(),
+  totalTime: z.number().int(),
+  submissionInfo: z.record(z.string(), z.unknown()),
+  contestId: z.number().int(),
+})
+
+export const contestRankSchema = paginatedSchema(contestRankItemSchema)