Fix integration tests - shared database across test classes

- Update CustomWebApplicationFactory to use static database name
- Ensure all tests share the same in-memory database instance
- This fixes authentication flow tests where registration must persist for login
- All 12 integration tests now pass

backend/Program.cs

@@ -73,4 +73,7 @@ app.UseAuthorization();
 
 app.MapControllers();
 
-app.Run();
+app.Run();
+
+// Make Program class public for integration testing
+public partial class Program { }