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

src/PLib.Application/Abstractions/IThumbnailGenerator.cs

@@ -0,0 +1,15 @@
+namespace PLib.Application.Abstractions;
+
+/// <summary>Produces (and caches on disk) a poster frame for a video file.</summary>
+public interface IThumbnailGenerator
+{
+    /// <summary>
+    /// Returns the absolute path of the poster frame for <paramref name="videoPath"/>,
+    /// generating it if it is not cached yet. Returns <c>null</c> when no frame could be
+    /// extracted; a missing thumbnail is a normal outcome, not an error.
+    /// </summary>
+    Task<string?> GetOrCreateAsync(
+        string videoPath,
+        TimeSpan? duration,
+        CancellationToken cancellationToken = default);
+}