perf(push-notification): optimize push notification dispatch by grouping device tokens

This commit improves the performance of the push notification dispatch process
by grouping device tokens by user ID before sending notifications. This change
avoids iterating through all devices for each user, which reduces the time
complexity from O(N^2) to O(N), significantly speeding up the dispatch process.

Author: fulleni

lib/src/services/push_notification_service.dart

@@ -187,6 +187,21 @@ class DefaultPushNotificationService implements IPushNotificationService {
         'Found ${targetedDevices.length} devices to target via $primaryProvider.',
       );
 
+      // 6. Group device tokens by user ID for efficient lookup.
+      // This avoids iterating through all devices for each user.
+      final userDeviceTokensMap = <String, List<String>>{};
+      for (final device in targetedDevices) {
+        final token = device.providerTokens[primaryProvider];
+        if (token != null) {
+          // If the user's list doesn't exist, create it.
+          if (!userDeviceTokensMap.containsKey(device.userId)) {
+            userDeviceTokensMap[device.userId] = [];
+          }
+          // Add the token to the user's list.
+          userDeviceTokensMap[device.userId]!.add(token);
+        }
+      }
+
       // 7. Iterate through each subscribed user to create and send a
       // personalized notification.
       final sendFutures = <Future<void>>[];
@@ -214,11 +229,8 @@ class DefaultPushNotificationService implements IPushNotificationService {
         try {
           await _inAppNotificationRepository.create(item: notification);
 
-          // Find all device tokens for the current user.
-          final userDeviceTokens = targetedDevices
-              .where((d) => d.userId == userId)
-              .map((d) => d.providerTokens[primaryProvider]!)
-              .toList();
+          // Efficiently retrieve the tokens for the current user from the map.
+          final userDeviceTokens = userDeviceTokensMap[userId] ?? [];
 
           if (userDeviceTokens.isNotEmpty) {
             // Add the send operation to the list of futures.