Implement base controller for shared functionality; update existing controllers to inherit from BaseController and adjust user ID handling. Add JWT authentication support and modify item retrieval methods in GameDataController.

Controllers/BaseController.cs

@@ -0,0 +1,13 @@
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+
+namespace PetCompanion.Controllers
+{
+    [Authorize]
+    [ApiController]
+    [Route("api/v1/[controller]")]
+    public class BaseController : Controller
+    {
+        protected string userId => User.Claims.FirstOrDefault(c => c.Type == "user_id").Value;
+    }
+}