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

crates/adapters/postgres/src/notification.rs

@@ -100,6 +100,17 @@ impl NotificationRepository for PgNotificationRepository {
         })
     }
 
+    async fn count_unread(&self, user_id: &UserId) -> Result<u64, DomainError> {
+        let count: i64 = sqlx::query_scalar(
+            "SELECT COUNT(*) FROM notifications WHERE user_id=$1 AND read=false",
+        )
+        .bind(user_id.as_uuid())
+        .fetch_one(&self.pool)
+        .await
+        .map_err(|e| DomainError::Internal(e.to_string()))?;
+        Ok(count as u64)
+    }
+
     async fn mark_read(&self, id: &NotificationId, user_id: &UserId) -> Result<(), DomainError> {
         sqlx::query("UPDATE notifications SET read=true WHERE id=$1 AND user_id=$2")
             .bind(id.as_uuid())