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/components/layout/AppSidebar.vue

@@ -319,6 +319,21 @@ const ServerIcon = {
     )
 }
 
+const BellIcon = {
+  render: () =>
+    h(
+      'svg',
+      { fill: 'none', viewBox: '0 0 24 24', stroke: 'currentColor', 'stroke-width': '1.5' },
+      [
+        h('path', {
+          'stroke-linecap': 'round',
+          'stroke-linejoin': 'round',
+          d: 'M14.857 17.082a23.848 23.848 0 005.454-1.31A8.967 8.967 0 0118 9.75V9a6 6 0 10-12 0v.75a8.967 8.967 0 01-2.312 6.022c1.733.64 3.56 1.085 5.455 1.31m5.714 0a24.255 24.255 0 01-5.714 0m5.714 0a3 3 0 11-5.714 0'
+        })
+      ]
+    )
+}
+
 const TicketIcon = {
   render: () =>
     h(
@@ -418,6 +433,7 @@ const ChevronDoubleRightIcon = {
 const userNavItems = computed(() => {
   const items = [
     { path: '/dashboard', label: t('nav.dashboard'), icon: DashboardIcon },
+    { path: '/announcements', label: t('nav.announcements'), icon: BellIcon },
     { path: '/keys', label: t('nav.apiKeys'), icon: KeyIcon },
     { path: '/usage', label: t('nav.usage'), icon: ChartIcon, hideInSimpleMode: true },
     { path: '/subscriptions', label: t('nav.mySubscriptions'), icon: CreditCardIcon, hideInSimpleMode: true },
@@ -440,6 +456,7 @@ const userNavItems = computed(() => {
 // Personal navigation items (for admin's "My Account" section, without Dashboard)
 const personalNavItems = computed(() => {
   const items = [
+    { path: '/announcements', label: t('nav.announcements'), icon: BellIcon },
     { path: '/keys', label: t('nav.apiKeys'), icon: KeyIcon },
     { path: '/usage', label: t('nav.usage'), icon: ChartIcon, hideInSimpleMode: true },
     { path: '/subscriptions', label: t('nav.mySubscriptions'), icon: CreditCardIcon, hideInSimpleMode: true },
@@ -470,6 +487,7 @@ const adminNavItems = computed(() => {
     { path: '/admin/groups', label: t('nav.groups'), icon: FolderIcon, hideInSimpleMode: true },
     { path: '/admin/subscriptions', label: t('nav.subscriptions'), icon: CreditCardIcon, hideInSimpleMode: true },
     { path: '/admin/accounts', label: t('nav.accounts'), icon: GlobeIcon },
+    { path: '/admin/announcements', label: t('nav.announcements'), icon: BellIcon },
     { path: '/admin/proxies', label: t('nav.proxies'), icon: ServerIcon },
     { path: '/admin/redeem', label: t('nav.redeemCodes'), icon: TicketIcon, hideInSimpleMode: true },
     { path: '/admin/promo-codes', label: t('nav.promoCodes'), icon: GiftIcon, hideInSimpleMode: true },