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/questions/questions.schema.ts

@@ -0,0 +1,23 @@
+import {
+  mysqlTable,
+  char,
+  varchar,
+  text,
+  int,
+  timestamp,
+} from "drizzle-orm/mysql-core";
+
+export const questions = mysqlTable("content_questions", {
+  id: char("id", { length: 36 }).notNull().primaryKey(),
+  knowledgePointId: char("knowledge_point_id", { length: 36 }).notNull(),
+  type: varchar("type", { length: 50 }).notNull(),
+  content: text("content").notNull(),
+  answer: text("answer"),
+  explanation: text("explanation"),
+  difficulty: int("difficulty").default(3),
+  createdAt: timestamp("created_at").notNull().defaultNow(),
+  updatedAt: timestamp("updated_at").notNull().defaultNow().onUpdateNow(),
+});
+
+export type Question = typeof questions.$inferSelect;
+export type NewQuestion = typeof questions.$inferInsert;