feat: implement POST /items endpoint for creating media items

- Add validation with @Valid for automatic error handling
- Convert MediaItemRequest to MediaItem using existing helper
- Add items to library via librarian authorization
- Return created item in response with proper status codes

Author: lquinoa252

lesson_23/api/java/api_app/src/main/java/com/codedifferently/lesson23/web/MediaItemsController.java

@@ -8,10 +8,13 @@
 import java.util.List;
 import java.util.Set;
 import java.util.UUID;
+import jakarta.validation.Valid;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.CrossOrigin;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RestController;
 
 @RestController
@@ -51,4 +54,21 @@ public ResponseEntity<MediaItemResponse> getItem(@PathVariable String id) {
       return ResponseEntity.badRequest().build();
     }
   }
+
+  @PostMapping("/items")
+  public ResponseEntity<CreateMediaItemResponse> createItem(@Valid @RequestBody CreateMediaItemRequest request) {
+    try {
+      MediaItem newItem = MediaItemRequest.asMediaItem(request.getItem());
+      library.addMediaItem(newItem, librarian);
+      
+      MediaItemResponse itemResponse = MediaItemResponse.from(newItem);
+      CreateMediaItemResponse response = CreateMediaItemResponse.builder()
+          .item(itemResponse)
+          .build();
+      
+      return ResponseEntity.ok(response);
+    } catch (Exception e) {
+      return ResponseEntity.badRequest().build();
+    }
+  }
 }