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

src/PLib.Domain/PLib.Domain.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <RootNamespace>PLib.Domain</RootNamespace>
+  </PropertyGroup>
+
+</Project>

src/PLib.Domain/Videos/VideoItem.cs

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+namespace PLib.Domain.Videos;
+
+/// <summary>
+/// A single video file that belongs to the library.
+/// </summary>
+/// <remarks>
+/// The absolute path is the natural identity of a video: the library is a view over the
+/// file system, so two entries pointing at the same path are the same video. Mutation goes
+/// through explicit methods so that the entity can never end up half-updated.
+/// </remarks>
+public sealed class VideoItem
+{
+    /// <summary>Required by EF Core materialization; do not use from application code.</summary>
+    private VideoItem()
+    {
+        FullPath = null!;
+        Title = null!;
+    }
+
+    public VideoItem(string fullPath, string title, long sizeInBytes, DateTimeOffset fileModifiedAt)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(fullPath);
+        ArgumentException.ThrowIfNullOrWhiteSpace(title);
+        ArgumentOutOfRangeException.ThrowIfNegative(sizeInBytes);
+
+        Id = Guid.CreateVersion7();
+        FullPath = fullPath;
+        Title = title;
+        SizeInBytes = sizeInBytes;
+        FileModifiedAt = fileModifiedAt;
+        AddedAt = DateTimeOffset.UtcNow;
+    }
+
+    public Guid Id { get; private set; }
+
+    /// <summary>Absolute path of the file on disk. Unique within the library.</summary>
+    public string FullPath { get; private set; }
+
+    /// <summary>Human readable name; defaults to the file name without extension.</summary>
+    public string Title { get; private set; }
+
+    public long SizeInBytes { get; private set; }
+
+    public TimeSpan? Duration { get; private set; }
+
+    public int? Width { get; private set; }
+
+    public int? Height { get; private set; }
+
+    public string? VideoCodec { get; private set; }
+
+    /// <summary>Absolute path of the generated poster frame, or <c>null</c> if none exists yet.</summary>
+    public string? ThumbnailPath { get; private set; }
+
+    /// <summary>Last write time of the file when it was last indexed.</summary>
+    public DateTimeOffset FileModifiedAt { get; private set; }
+
+    public DateTimeOffset AddedAt { get; private set; }
+
+    /// <summary>True once the file has been probed and a poster frame produced.</summary>
+    public bool IsIndexed => Duration is not null && ThumbnailPath is not null;
+
+    public void Rename(string title)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(title);
+        Title = title;
+    }
+
+    public void ApplyTechnicalInfo(VideoTechnicalInfo info)
+    {
+        Duration = info.Duration;
+        Width = info.Width;
+        Height = info.Height;
+        VideoCodec = info.VideoCodec;
+    }
+
+    public void AttachThumbnail(string thumbnailPath)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(thumbnailPath);
+        ThumbnailPath = thumbnailPath;
+    }
+
+    public void DetachThumbnail() => ThumbnailPath = null;
+
+    /// <summary>
+    /// Refreshes the file system facts after the file changed on disk, and invalidates
+    /// everything that was derived from the previous revision of the file.
+    /// </summary>
+    public void RefreshFileFacts(long sizeInBytes, DateTimeOffset fileModifiedAt)
+    {
+        ArgumentOutOfRangeException.ThrowIfNegative(sizeInBytes);
+
+        if (SizeInBytes == sizeInBytes && FileModifiedAt == fileModifiedAt)
+        {
+            return;
+        }
+
+        SizeInBytes = sizeInBytes;
+        FileModifiedAt = fileModifiedAt;
+        ApplyTechnicalInfo(VideoTechnicalInfo.Unknown);
+        DetachThumbnail();
+    }
+}

src/PLib.Domain/Videos/VideoTechnicalInfo.cs

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+namespace PLib.Domain.Videos;
+
+/// <summary>
+/// Technical facts about a media file that are discovered by probing it,
+/// as opposed to facts we already know from the file system entry itself.
+/// </summary>
+/// <param name="Duration">Playback duration, or <c>null</c> if the container did not report one.</param>
+/// <param name="Width">Width of the primary video stream in pixels.</param>
+/// <param name="Height">Height of the primary video stream in pixels.</param>
+/// <param name="VideoCodec">Short codec name of the primary video stream, e.g. <c>h264</c>.</param>
+public readonly record struct VideoTechnicalInfo(
+    TimeSpan? Duration,
+    int? Width,
+    int? Height,
+    string? VideoCodec)
+{
+    public static VideoTechnicalInfo Unknown => default;
+}