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>