refactor(db): 精简 generatedFields，删三分支 PK 策略与泛型 keys 工具

- PK 策略原本给了 native(PG18)/extension/app-side 三条路，对 base 项目
  是 YAGNI。全部落到最稳妥的 $defaultFn(uuidv7)：任意 PG 版本都能跑，
  不依赖扩展或 18+ 的 uuidv7() 原生函数
- createGeneratedFieldKeys 泛型 reduce 只为了生成 { id: true, createdAt:
  true, updatedAt: true } 这三项，直接手写 as const 更直观
- 删掉 pk/id/createdAt/updatedAt 的独立 helper 导出——没人引用，它们
  只是 generatedFields 的内部组合

src/server/db/fields.ts

@@ -1,55 +1,15 @@
-import { sql } from 'drizzle-orm'
 import { timestamp, uuid } from 'drizzle-orm/pg-core'
 import { v7 as uuidv7 } from 'uuid'
 
-// id
-
-const id = (name: string) => uuid(name)
-export const pk = (name: string, strategy?: 'native' | 'extension') => {
-  switch (strategy) {
-    // PG 18+
-    case 'native':
-      return id(name).primaryKey().default(sql`uuidv7()`)
-
-    // PG 13+ with extension
-    case 'extension':
-      return id(name).primaryKey().default(sql`uuid_generate_v7()`)
-
-    // Any PG version
-    default:
-      return id(name)
-        .primaryKey()
-        .$defaultFn(() => uuidv7())
-  }
-}
-
-// timestamp
-
-export const createdAt = (name = 'created_at') => timestamp(name, { withTimezone: true }).notNull().defaultNow()
-
-export const updatedAt = (name = 'updated_at') =>
-  timestamp(name, { withTimezone: true })
+export const generatedFields = {
+  id: uuid('id')
+    .primaryKey()
+    .$defaultFn(() => uuidv7()),
+  createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
+  updatedAt: timestamp('updated_at', { withTimezone: true })
     .notNull()
     .defaultNow()
-    .$onUpdateFn(() => new Date())
-
-// generated fields
-
-export const generatedFields = {
-  id: pk('id'),
-  createdAt: createdAt('created_at'),
-  updatedAt: updatedAt('updated_at'),
+    .$onUpdateFn(() => new Date()),
 }
 
-// Helper to create omit keys from generatedFields
-const createGeneratedFieldKeys = <T extends Record<string, unknown>>(fields: T): Record<keyof T, true> => {
-  return Object.keys(fields).reduce(
-    (acc, key) => {
-      acc[key as keyof T] = true
-      return acc
-    },
-    {} as Record<keyof T, true>,
-  )
-}
-
-export const generatedFieldKeys = createGeneratedFieldKeys(generatedFields)
+export const generatedFieldKeys = { id: true, createdAt: true, updatedAt: true } as const