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/index.ts

@@ -16,6 +16,7 @@ export { userAPI } from './user'
 export { redeemAPI, type RedeemHistoryItem } from './redeem'
 export { userGroupsAPI } from './groups'
 export { totpAPI } from './totp'
+export { default as announcementsAPI } from './announcements'
 
 // Admin APIs
 export { adminAPI } from './admin'