feat(validation): record independent discovery votes during catalog adds

- Add vote tracking when users add an existing global car entry
- Guard against duplicate votes by checking user_id + hw_id first
- Keep new-discovery flow voting unchanged while extending repeated-add validation
- Align scanner business flow with TPB community verification model

lib/screens/scan_tab.dart

@@ -340,6 +340,7 @@ class ScanTabState extends State<ScanTab> {
 
         if (addConfirmed == true) {
           await _addToCollection(collection.id, hwId);
+          await _ensureValidationVote(hwId);
           if (!mounted) return false;
           showGlobalSnackBar('$hwId added to "${collection.name}"! 🎉');
         }
@@ -410,6 +411,23 @@ class ScanTabState extends State<ScanTab> {
       'user_id': supabase.auth.currentUser!.id,
     });
   }
+
+  Future<void> _ensureValidationVote(String hwId) async {
+    final userId = supabase.auth.currentUser!.id;
+    final existingVote = await supabase
+        .from('car_votes')
+        .select('id')
+        .eq('hw_id', hwId)
+        .eq('user_id', userId)
+        .maybeSingle();
+
+    if (existingVote != null) return;
+
+    await supabase.from('car_votes').insert({
+      'hw_id': hwId,
+      'user_id': userId,
+    });
+  }
 }
 
 class _FoundCarSheet extends StatelessWidget {