TASK 21 Increment A: media-source model + persistence (schema migration + round-trip test)

SourceType.MediaSource added; MediaSourceType (Video/Audio) and MediaPlaybackState
(Stopped/Playing/Paused) enums. Source gains MediaPath, MediaIsLooping, MediaVolume
(0-1 clamped), MediaPlaybackState. LayoutStore: MigrateSourceTable adds 4 guarded
ALTERs (MediaPath, MediaIsLooping, MediaVolume, MediaPlaybackState); SELECT appends
indices 26-29, INSERT adds 4 columns + params. Round-trip test
MediaSource_Config_Survives_Save_And_Reload passes. Build 0 warnings, 245/247
tests (2 known pre-existing). Gated on creator run for later integration (ffmpeg).

Models/Source.cs

@@ -12,7 +12,23 @@ public enum SourceType
     TextOverlay,
     Countdown,
     WebSource,
-    ChatBox
+    ChatBox,
+    MediaSource
+}
+
+/// <summary>Distinguishes how a media source presents its content (TASK 21).</summary>
+public enum MediaSourceType
+{
+    Video,
+    Audio
+}
+
+/// <summary>Playback state of a media source (TASK 21).</summary>
+public enum MediaPlaybackState
+{
+    Stopped,
+    Playing,
+    Paused
 }
 
 public enum ClipShape
@@ -113,6 +129,48 @@ public class Source : SceneElement
     private string? _webUri;
     public string? WebUri { get => _webUri; set => Set(ref _webUri, value); }
 
+    // Media source config (TASK 21: video/audio file playback)
+    private MediaSourceType _mediaType = MediaSourceType.Video;
+    public MediaSourceType MediaType
+    {
+        get => _mediaType;
+        set => Set(ref _mediaType, value);
+    }
+
+    private string? _mediaPath;
+    public string? MediaPath
+    {
+        get => _mediaPath;
+        set
+        {
+            if (Set(ref _mediaPath, value))
+                Raise(nameof(IsMediaSource));
+        }
+    }
+
+    private bool _mediaIsLooping;
+    public bool MediaIsLooping
+    {
+        get => _mediaIsLooping;
+        set => Set(ref _mediaIsLooping, value);
+    }
+
+    private double _mediaVolume = 1.0;
+    public double MediaVolume
+    {
+        get => _mediaVolume;
+        set => Set(ref _mediaVolume, Math.Clamp(value, 0.0, 1.0));
+    }
+
+    private MediaPlaybackState _mediaPlaybackState = MediaPlaybackState.Stopped;
+    public MediaPlaybackState MediaPlaybackState
+    {
+        get => _mediaPlaybackState;
+        set => Set(ref _mediaPlaybackState, value);
+    }
+
+    public bool IsMediaSource => Type == SourceType.MediaSource;
+
     // Chat box config (TASK 3.18)
     private double _chatFontSize = 16;
     public double ChatFontSize