docs: Add library folder validation to Create Media Item Bruno test

- Add test_create_media_item_no_folders test case
- Document 400 error for libraries without folders
- Add prerequisites section explaining folder requirement
- Include setup workflow example
- Link to Add Library Folder Bruno request

Related: Handler validation and service layer changes

bruno/media-items/Create Media Item.bru

@@ -62,6 +62,33 @@ tests {
 
     return true;
   }
+
+  test_create_media_item_no_folders(status, headers, body) {
+    if (status !== 400) {
+      throw new Error("Expected status 400 for library with no folders, got " + status);
+    }
+
+    let data;
+    try {
+      data = JSON.parse(body);
+    } catch (e) {
+      throw new Error("Response body is not valid JSON");
+    }
+
+    if (!data || typeof data !== 'object') {
+      throw new Error("Expected error object in response");
+    }
+
+    if (!data.error || typeof data.error !== 'string') {
+      throw new Error("Expected error message in response");
+    }
+
+    if (!data.error.includes("folder")) {
+      throw new Error("Error message should mention folders requirement");
+    }
+
+    return true;
+  }
 }
 
 vars:pre-request {
@@ -84,6 +111,11 @@ settings {
    
    **Authentication:** Required (Bearer token, admin only)
    
+   **Prerequisites**
+   - Library must have at least one folder configured before media items can be added
+   - Use `POST /api/libraries/{library_id}/folders` to add folders first
+   - See: [Add Library Folder](../../library/Add%20Library%20Folder.bru)
+   
    **Request Body:**
    - `library_id` (string, required): Library UUID
    - `title` (string, required): Media item title
@@ -127,14 +159,37 @@ settings {
    - Case-insensitive deduplication (keeps version with punctuation if exists)
    - Example: `["acme corp", "ACME CORP.", "acme corp"]` → `["ACME CORP."]`
    
+   **Validation Error Response**
+   - **400 Bad Request** - Library has no folders:
+     ```json
+     {
+       "error": "Cannot add media items to a library with no folders. Please add at least one folder to the library first."
+     }
+     ```
+   
    **Status Codes:**
    - 201: Media item created successfully
-   - 400: Invalid request data
+   - 400: Invalid request data OR library has no folders
    - 401: Unauthorized
    - 403: Forbidden (admin access required)
    - 404: Library not found
    - 500: Internal server error
    
+   **Setup Workflow Example:**
+   ```bash
+   # 1. Create library
+   POST /api/libraries
+   { "name": "My Books", "type": "ebooks" }
+   
+   # 2. Add folder to library (REQUIRED before adding media items)
+   POST /api/libraries/{library_id}/folders
+   { "folder_path": "/mnt/books/my-library" }
+   
+   # 3. Create media items (now that library has folders)
+   POST /api/media-items
+   { "library_id": "...", "title": "Book Title", ... }
+   ```
+   
    **Examples:**
    - Create media item: `POST /api/media-items`