feat(songs): 歌曲管理支持批量驳回、投稿留言自动审查与排期范围可见性过滤

- 新增 /api/admin/songs/batch-reject.post 批量驳回接口（事务清排期/投票/扣接纳数/黑名单去重/异步通知）
- requireSongAdmin 三角色校验 + createApiError 统一错误包装
- scheduleDaysBefore/After 四字段排期可见范围限制（开放 API 与公开排期接口三处缓存路径 + DB 条件过滤）
- submissionNoteAutoModeration 基于黑名单 KEYWORD/LANGUAGE/GENRE 自动判定留言 publicStatus，命中自动关闭 submissionNotePublic
- 更新 songs/request.post 与 songs/[id]/update.put 接入自动审查逻辑

server/utils/important-notification-policy.ts

@@ -0,0 +1,72 @@
+export const NOTIFICATION_TITLE_MAX_LENGTH = 200
+export const NOTIFICATION_CONTENT_MAX_LENGTH = 20000
+export const NOTIFICATION_SOURCES = {
+  SYSTEM: 'SYSTEM',
+  ADMIN_MANUAL: 'ADMIN_MANUAL'
+} as const
+
+export type NotificationSenderInput = {
+  id: number
+  name?: string | null
+  username?: string | null
+}
+
+export type NotificationSenderSnapshot = {
+  senderId: number | null
+  senderName: string | null
+  senderUsername: string | null
+}
+
+export const resolveNotificationSource = (sender: NotificationSenderInput | null) =>
+  sender ? NOTIFICATION_SOURCES.ADMIN_MANUAL : NOTIFICATION_SOURCES.SYSTEM
+
+export const createNotificationSenderSnapshot = (
+  sender: NotificationSenderInput | null = null
+): NotificationSenderSnapshot => ({
+  senderId: sender?.id ?? null,
+  senderName: sender?.name?.trim() || null,
+  senderUsername: sender?.username?.trim() || null
+})
+
+export const serializeNotificationSender = (snapshot: NotificationSenderSnapshot) =>
+  snapshot.senderId
+    ? {
+        id: snapshot.senderId,
+        name: snapshot.senderName,
+        username: snapshot.senderUsername
+      }
+    : null
+
+export const shouldCheckImportantNotification = (
+  authenticated: boolean,
+  userId?: number | null,
+  requirePasswordChange = false
+): boolean =>
+  authenticated && Number.isInteger(userId) && Number(userId) > 0 && !requirePasswordChange
+
+export const canSendSystemNotification = (role?: string): boolean =>
+  role === 'ADMIN' || role === 'SUPER_ADMIN'
+
+export const resolveImportantFlag = (value: unknown): boolean | null => {
+  if (value === undefined) return false
+  return typeof value === 'boolean' ? value : null
+}
+
+export const shouldDeliverSystemNotification = (
+  important: boolean,
+  notificationsEnabled?: boolean
+): boolean => important || notificationsEnabled !== false
+
+export const shouldRetainNotificationHistory = (type: string, source: string): boolean =>
+  type === 'SYSTEM_NOTICE' && source === NOTIFICATION_SOURCES.ADMIN_MANUAL
+
+export const createNotificationReadUpdate = (updatedAt = new Date()) => ({
+  read: true as const,
+  updatedAt
+})
+
+export const createNotificationUserDeleteUpdate = (updatedAt = new Date()) => ({
+  userDeleted: true as const,
+  read: true as const,
+  updatedAt
+})