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/content.ts

@@ -0,0 +1,74 @@
+import { z } from "zod"
+
+import { paginatedSchema, sampleUserSchema } from "./common"
+import { submissionDetailSchema } from "./submission"
+
+export const announcementSchema = z.object({
+  id: z.number().int(),
+  title: z.string(),
+  tag: z.string(),
+  content: z.string().optional(),
+  top: z.boolean(),
+  createdBy: sampleUserSchema,
+  createTime: z.string(),
+  lastUpdateTime: z.string(),
+})
+
+export const announcementListSchema = paginatedSchema(announcementSchema)
+
+export const messageSchema = z.object({
+  id: z.number().int(),
+  sender: sampleUserSchema,
+  createTime: z.string(),
+  message: z.string(),
+  submission: submissionDetailSchema,
+})
+
+export const messageListSchema = paginatedSchema(messageSchema)
+
+export const createMessageRequestSchema = z.object({
+  recipientId: z.number().int().positive(),
+  submissionId: z.string().min(1),
+  message: z.string().min(1).max(1024 * 1024),
+})
+
+export const reactionKeySchema = z.enum([
+  "too_easy",
+  "too_hard",
+  "confusing",
+  "buggy",
+  "learned",
+  "interesting",
+  "want_explain",
+])
+
+export const reactionCountsSchema = z.record(reactionKeySchema, z.number().int())
+export const reactionStateSchema = z.object({
+  mine: reactionKeySchema.nullable(),
+  counts: reactionCountsSchema.nullable(),
+})
+
+export const setReactionRequestSchema = z.object({ type: reactionKeySchema })
+
+export const tutorialSummarySchema = z.object({
+  id: z.number().int(),
+  title: z.string(),
+})
+
+export const tutorialSchema = tutorialSummarySchema.extend({
+  content: z.string(),
+  code: z.string().nullable(),
+  isPublic: z.boolean(),
+  order: z.number().int(),
+  type: z.enum(["python", "c"]),
+  createdBy: sampleUserSchema,
+  createdAt: z.string(),
+  updatedAt: z.string(),
+})
+
+export const exerciseSchema = z.object({
+  id: z.number().int(),
+  type: z.enum(["mcq", "sort", "fill", "match", "predict", "debug", "group"]),
+  data: z.record(z.string(), z.unknown()),
+  order: z.number().int(),
+})