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

crates/domain/src/testing.rs

@@ -498,6 +498,15 @@ impl NotificationRepository for TestStore {
             per_page: 20,
         })
     }
+    async fn count_unread(&self, uid: &UserId) -> Result<u64, DomainError> {
+        Ok(self
+            .notifications
+            .lock()
+            .unwrap()
+            .iter()
+            .filter(|n| &n.user_id == uid && !n.read)
+            .count() as u64)
+    }
     async fn mark_read(&self, id: &NotificationId, _uid: &UserId) -> Result<(), DomainError> {
         if let Some(n) = self
             .notifications