feat(announcements): add admin/user announcement system

Implements announcements end-to-end (admin CRUD + read status, user list + mark read) with OR-of-AND targeting. Also breaks the ent<->service import cycle by moving schema-facing constants/targeting into a new domain package.

frontend/src/api/announcements.ts

@@ -0,0 +1,26 @@
+/**
+ * User Announcements API endpoints
+ */
+
+import { apiClient } from './client'
+import type { UserAnnouncement } from '@/types'
+
+export async function list(unreadOnly: boolean = false): Promise<UserAnnouncement[]> {
+  const { data } = await apiClient.get<UserAnnouncement[]>('/announcements', {
+    params: unreadOnly ? { unread_only: 1 } : {}
+  })
+  return data
+}
+
+export async function markRead(id: number): Promise<{ message: string }> {
+  const { data } = await apiClient.post<{ message: string }>(`/announcements/${id}/read`)
+  return data
+}
+
+const announcementsAPI = {
+  list,
+  markRead
+}
+
+export default announcementsAPI
+