fix: Replace COUNT(*) with SELECT id in FOR UPDATE query (refs #23)

PostgreSQL error 0A000 (feature_not_supported) occurs when using
FOR UPDATE with aggregate functions like COUNT(*). Row-level locking
requires actual rows to lock.

Changes:
- Select id column instead of COUNT(*) aggregate
- Count rows in application using .length
- Maintains transaction isolation and race condition prevention

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/features/vehicles/domain/vehicles.service.ts

@@ -86,11 +86,12 @@ export class VehiclesService {
       await client.query('BEGIN');
 
       // Lock user's vehicle rows and get count
-      const countResult = await client.query(
+      // Note: Cannot use COUNT(*) with FOR UPDATE, so we select IDs and count in app
-        'SELECT COUNT(*) as count FROM vehicles WHERE user_id = $1 AND is_active = true FOR UPDATE',
+      const lockResult = await client.query(
+        'SELECT id FROM vehicles WHERE user_id = $1 AND is_active = true FOR UPDATE',
         [userId]
       );
-      const currentCount = parseInt(countResult.rows[0].count, 10);
+      const currentCount = lockResult.rows.length;
 
       // Check if user can add another vehicle
       if (!canAddVehicle(userTier, currentCount)) {