feat(api): GET /api/collections/{id}/items (paged) + confirm POST-items 422 guard (#155)

Adds a paged collection-items endpoint reusing LibraryBrowseItemResponseModel
so the SPA lists a manual collection's full contents (all media kinds), replacing
the lossy Lucene name-based preview. Confirms POST /items already returns 422 for
bogus ids (guarded by ValidateMediaItems, fb3f2856); adds endpoint-level coverage.

fixes #155

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV/Controllers/Api/CollectionController.cs

@@ -2,6 +2,7 @@ using System.ComponentModel.DataAnnotations;
 using ErsatzTV.Application.MediaCollections;
 using ErsatzTV.Controllers.Api.Requests;
 using ErsatzTV.Core;
+using ErsatzTV.Core.Api.LibraryBrowse;
 using ErsatzTV.Extensions;
 using MediatR;
 using Microsoft.AspNetCore.Http;
@@ -32,6 +33,28 @@ public class CollectionController(IMediator mediator) : ControllerBase
         return result.ToGetResult();
     }
 
+    [HttpGet("/api/collections/{id:int}/items", Name = "GetCollectionItems")]
+    [Tags("Collections")]
+    [EndpointSummary("Get the items in a manual collection")]
+    [EndpointDescription("Returns a manual collection's full contents (all media kinds), paged.")]
+    [EndpointGroupName("general")]
+    [ProducesResponseType(typeof(PagedLibraryBrowseItemsResponseModel), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status404NotFound)]
+    public async Task<IActionResult> GetItems(
+        int id,
+        [FromQuery] int pageNum = 0,
+        [FromQuery] int pageSize = 100,
+        CancellationToken cancellationToken = default)
+    {
+        int clampedPageNum = Math.Max(0, pageNum);
+        int clampedPageSize = Math.Clamp(pageSize, 1, 100);
+
+        Either<BaseError, PagedLibraryBrowseItemsResponseModel> result = await mediator.Send(
+            new GetCollectionItems(id, clampedPageNum, clampedPageSize),
+            cancellationToken);
+        return result.ToUpdatedResult();
+    }
+
     [HttpPost("/api/collections")]
     [Tags("Collections")]
     [EndpointSummary("Create a collection")]