- Add CoverAssetId FK and CoverAsset navigation to Album model
- Add AlbumAssetPreviewDto with resolution/thumbnail info
- Extend AlbumPreviewDto/FullDto with PersonName, CoverAssetId, AssetCount
- Add CoverAssetId to AlbumCreateDto and AlbumUpdateDto
- Update AlbumMapper to populate new fields from eager-loaded relations
- Add EF migration for CoverAssetId column