refactor: 重构数据库实例管理，支持按需创建与上下文注入

- 移除默认导出的 db 实例，改为按需创建数据库实例
- 使用数据库中间件提供上下文中的数据库实例，统一通过上下文获取数据库连接并执行操作。
- 添加数据库中间件以在请求上下文中注入数据库实例，支持服务器端全局缓存和无服务器环境下的每次新建实例。
- 导出中间件模块中的数据库相关功能。

src/orpc/handlers/todo.ts

@@ -6,8 +6,8 @@ import {
   createUpdateSchema,
 } from 'drizzle-zod'
 import { z } from 'zod'
-import { db } from '@/db'
 import { todoTable } from '@/db/schema'
+import { dbProvider } from '../middlewares'
 
 const selectSchema = createSelectSchema(todoTable)
 
@@ -26,8 +26,9 @@ const updateSchema = createUpdateSchema(todoTable).omit({
 export const list = os
   .input(z.void())
   .output(z.array(selectSchema))
-  .handler(async () => {
-    const todos = await db.query.todoTable.findMany({
+  .use(dbProvider)
+  .handler(async ({ context }) => {
+    const todos = await context.db.query.todoTable.findMany({
       orderBy: (todos, { desc }) => [desc(todos.createdAt)],
     })
     return todos
@@ -36,10 +37,11 @@ export const list = os
 export const create = os
   .input(insertSchema)
   .output(selectSchema)
-  .handler(async ({ input }) => {
-    const [newTodo] = await db
+  .use(dbProvider)
+  .handler(async ({ context, input }) => {
+    const [newTodo] = await context.db
       .insert(todoTable)
-      .values({ title: input.title })
+      .values(input)
       .returning()
 
     if (!newTodo) {
@@ -57,8 +59,9 @@ export const update = os
     }),
   )
   .output(selectSchema)
-  .handler(async ({ input }) => {
-    const [updatedTodo] = await db
+  .use(dbProvider)
+  .handler(async ({ context, input }) => {
+    const [updatedTodo] = await context.db
       .update(todoTable)
       .set(input.data)
       .where(eq(todoTable.id, input.id))
@@ -78,6 +81,7 @@ export const remove = os
     }),
   )
   .output(z.void())
-  .handler(async ({ input }) => {
-    await db.delete(todoTable).where(eq(todoTable.id, input.id))
+  .use(dbProvider)
+  .handler(async ({ context, input }) => {
+    await context.db.delete(todoTable).where(eq(todoTable.id, input.id))
   })