Fix Linux background sync service code review issues

- Fix subprocess_helper_command_str to use Process.spawn_command_line_sync (line 148)
- Improve comment for fetch_subscriptions_needing_sync placeholder (line 303)

These fixes address issues identified in code review for FRE-531.

native-route/linux/src/background-sync.vala

@@ -145,8 +145,18 @@ public class BackgroundSyncService : Object {
     public bool are_background_tasks_enabled() {
         // Check if systemd timer is active
         try {
-            var result = subprocess_helper_command_str(
-                "systemctl", "is-enabled", "rssuper-sync.timer");
+            var stdout, stderr;
+            var exit_status = Process.spawn_command_line_sync(
+                "systemctl is-enabled rssuper-sync.timer",
+                out stdout, out stderr
+            );
+            
+            if (exit_status != 0) {
+                // Timer might not be installed
+                return true;
+            }
+            
+            var result = stdout.to_string();
             return result.strip() == "enabled";
         } catch (Error e) {
             // Timer might not be installed
@@ -302,7 +312,8 @@ public class SyncWorker : Object {
      */
     private List<Subscription> fetch_subscriptions_needing_sync() {
         // TODO: Replace with actual database query
-        // For now, return empty list as placeholder
+        // This is a placeholder that returns an empty list until the database layer is implemented
+        // Once database is available, query subscriptions where last_sync_at + interval < now
         return new List<Subscription>();
     }