Update README.md with project details, features, requirements, architecture, and data management for PLib video library manager.

src/PLib.Application/Library/ILibraryService.cs

@@ -0,0 +1,18 @@
+using PLib.Domain.Videos;
+
+namespace PLib.Application.Library;
+
+/// <summary>Use cases the UI needs in order to show and refresh the video library.</summary>
+public interface ILibraryService
+{
+    /// <summary>Everything currently stored in the library, newest first.</summary>
+    Task<IReadOnlyList<VideoItem>> GetLibraryAsync(CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Reconciles the library with the configured folders and then fills in metadata and
+    /// poster frames for anything that is missing them, streaming progress as it goes.
+    /// </summary>
+    IAsyncEnumerable<LibraryScanEvent> ScanAsync(
+        IReadOnlyList<string> folders,
+        CancellationToken cancellationToken = default);
+}