feat(content): 修复服务并添加chapters/knowledge-points/questions模块

- database.ts 导出db常量替代getDb()函数

- env.ts JWT_SECRET/ES_URL/NEO4J_URL改optional加DEV_MODE

- neo4j.ts driver惰性创建+try/catch+connectionTimeout:3000

- health/lifecycle改用Drizzle原生查询

- textbooks.schema修复integer到int+导出NewTextbook类型

- 新建chapters/knowledge-points/questions三模块CRUD

- knowledge-points含Neo4j前置依赖图非阻塞查询

- content-init.sql创建4张表

端到端验证: textbooks/chapters/knowledge-points/questions全CRUD通过

services/content/src/config/neo4j.ts

@@ -1,11 +1,38 @@
-import neo4j from 'neo4j-driver';
-import { env } from './env.js';
+import neo4j from "neo4j-driver";
+import type { Driver, Session } from "neo4j-driver";
+import { env } from "./env.js";
 
-export const neo4jDriver = neo4j.driver(
-  env.NEO4J_URL,
-  neo4j.auth.basic('neo4j', env.NEO4J_PASSWORD)
-);
+// Neo4j driver 创建为惰性初始化：未配置 NEO4J_URL / NEO4J_PASSWORD 时
+// driver 保持 null，服务仍可正常启动。所有依赖 Neo4j 的查询在
+// driver 为 null 时返回空结果或抛出可控错误，不会阻塞主流程。
+let driver: Driver | null = null;
+
+try {
+  if (env.NEO4J_URL && env.NEO4J_PASSWORD) {
+    driver = neo4j.driver(
+      env.NEO4J_URL,
+      neo4j.auth.basic("neo4j", env.NEO4J_PASSWORD),
+      // 连接超时 3s：Neo4j 不可用时快速失败，避免拖慢 HTTP 响应
+      { connectionTimeout: 3000, maxConnectionLifetime: 60_000 },
+    );
+  }
+} catch (err) {
+  console.warn(
+    "Neo4j driver init failed, running without Neo4j:",
+    err instanceof Error ? err.message : String(err),
+  );
+  driver = null;
+}
+
+export function getNeo4jSession(): Session | null {
+  if (!driver) {
+    return null;
+  }
+  return driver.session();
+}
 
 export async function closeNeo4j(): Promise<void> {
-  await neo4jDriver.close();
+  if (driver) {
+    await driver.close();
+  }
 }