feat: implement unread notification count and enhance user listing with pagination

crates/domain/src/ports.rs

@@ -184,6 +184,7 @@ pub trait NotificationRepository: Send + Sync {
         user_id: &UserId,
         page: &PageParams,
     ) -> Result<Paginated<Notification>, DomainError>;
+    async fn count_unread(&self, user_id: &UserId) -> Result<u64, DomainError>;
     async fn mark_read(&self, id: &NotificationId, user_id: &UserId) -> Result<(), DomainError>;
     async fn mark_all_read(&self, user_id: &UserId) -> Result<(), DomainError>;
 }