refactor(imports): #nitro 重命名为 #server，imports 字段 ASCII 排序

命名上避开实现耦合：被指向的 .output/server/index.mjs 本质是"编译后的 HTTP 服务器入口"，与 nitro（构建器）无关；包装器 _serve-nitro.mjs 仍持"nitro"。未来即便切换至 hono / vite-ssr / h3，#server 名字仍准确。

字段排序改为 ASCII 升序：#drizzle/*.sql < #package < #server，便于 sort-package-json 与人眼对账。

端到端验证（compose + Postgres 18）：
- ./server migrate ✓ (embedded SQL)
- /health = ok ✓
- /api/spec.json title=fullstack-starter version=1.0.0 ✓ (#package)
- RPC todo create+list 完整 round-trip ✓ (#server 解析 nitro 产物)
- biome/typecheck/test/build/compile 全绿

package.json

@@ -4,9 +4,9 @@
   "private": true,
   "type": "module",
   "imports": {
+    "#drizzle/*.sql": "./drizzle/*.sql",
     "#package": "./package.json",
-    "#nitro": "./.output/server/index.mjs",
-    "#drizzle/*.sql": "./drizzle/*.sql"
+    "#server": "./.output/server/index.mjs"
   },
   "scripts": {
     "build": "bunx --bun vite build",