refactor(api): 删掉 db ORPC middleware，handler 直接用 db

db middleware 的存在只是为了把 db 注入到 ORPC context——这是 Cloudflare
Workers / 多租户场景的模式（db 依赖 per-request 的 env binding）。在
Bun 单进程 + 模块级 const db 下，这层中间件是纯粹的仪式：一行 import
直接拿到 db，反而更清晰。

- 删除 src/server/api/middlewares/ 整个目录（不留空脚手架，KISS）
- context.ts 去掉 DBContext 与示例注释，只留 BaseContext { headers }
  作为未来 auth/tenant 等 middleware 的扩展点
- routers/todo.router.ts 不再 .use(db)，handler 内直接 db.query / db.insert

需要 per-request 上下文（auth、tenant、rate-limit）时再按 ORPC 的
os.middleware 模式新增，不在此预先铺陈。

src/server/api/routers/todo.router.ts

@@ -1,18 +1,17 @@
 import { ORPCError } from '@orpc/server'
 import { eq } from 'drizzle-orm'
-import { db } from '@/server/api/middlewares'
 import { os } from '@/server/api/server'
+import { db } from '@/server/db'
 import { todoTable } from '@/server/db/schema'
 
-export const list = os.todo.list.use(db).handler(async ({ context }) => {
-  const todos = await context.db.query.todoTable.findMany({
+export const list = os.todo.list.handler(async () => {
+  return db.query.todoTable.findMany({
     orderBy: (table, { desc }) => desc(table.createdAt),
   })
-  return todos
 })
 
-export const create = os.todo.create.use(db).handler(async ({ context, input }) => {
-  const [newTodo] = await context.db.insert(todoTable).values(input).returning()
+export const create = os.todo.create.handler(async ({ input }) => {
+  const [newTodo] = await db.insert(todoTable).values(input).returning()
 
   if (!newTodo) {
     throw new ORPCError('INTERNAL_SERVER_ERROR', { message: 'Failed to create todo' })
@@ -21,8 +20,8 @@ export const create = os.todo.create.use(db).handler(async ({ context, input })
   return newTodo
 })
 
-export const update = os.todo.update.use(db).handler(async ({ context, input }) => {
-  const [updatedTodo] = await context.db.update(todoTable).set(input.data).where(eq(todoTable.id, input.id)).returning()
+export const update = os.todo.update.handler(async ({ input }) => {
+  const [updatedTodo] = await db.update(todoTable).set(input.data).where(eq(todoTable.id, input.id)).returning()
 
   if (!updatedTodo) {
     throw new ORPCError('NOT_FOUND')
@@ -31,8 +30,8 @@ export const update = os.todo.update.use(db).handler(async ({ context, input })
   return updatedTodo
 })
 
-export const remove = os.todo.remove.use(db).handler(async ({ context, input }) => {
-  const [deleted] = await context.db.delete(todoTable).where(eq(todoTable.id, input.id)).returning({ id: todoTable.id })
+export const remove = os.todo.remove.handler(async ({ input }) => {
+  const [deleted] = await db.delete(todoTable).where(eq(todoTable.id, input.id)).returning({ id: todoTable.id })
 
   if (!deleted) {
     throw new ORPCError('NOT_FOUND')