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.

backend/ent/hook/hook.go

@@ -45,6 +45,30 @@ func (f AccountGroupFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value
 	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.AccountGroupMutation", m)
 }
 
+// The AnnouncementFunc type is an adapter to allow the use of ordinary
+// function as Announcement mutator.
+type AnnouncementFunc func(context.Context, *ent.AnnouncementMutation) (ent.Value, error)
+
+// Mutate calls f(ctx, m).
+func (f AnnouncementFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value, error) {
+	if mv, ok := m.(*ent.AnnouncementMutation); ok {
+		return f(ctx, mv)
+	}
+	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.AnnouncementMutation", m)
+}
+
+// The AnnouncementReadFunc type is an adapter to allow the use of ordinary
+// function as AnnouncementRead mutator.
+type AnnouncementReadFunc func(context.Context, *ent.AnnouncementReadMutation) (ent.Value, error)
+
+// Mutate calls f(ctx, m).
+func (f AnnouncementReadFunc) Mutate(ctx context.Context, m ent.Mutation) (ent.Value, error) {
+	if mv, ok := m.(*ent.AnnouncementReadMutation); ok {
+		return f(ctx, mv)
+	}
+	return nil, fmt.Errorf("unexpected mutation type %T. expect *ent.AnnouncementReadMutation", m)
+}
+
 // The GroupFunc type is an adapter to allow the use of ordinary
 // function as Group mutator.
 type GroupFunc func(context.Context, *ent.GroupMutation) (ent.Value, error)