fix(496): give music videos a per-library server identity; itemId diff + soft trash

Music videos carried no server identity, so JellyfinMusicVideoLibraryScanner had to
reconcile by a (LibraryPathId, path) diff and HARD-delete the remainder. A file served
by two libraries with overlapping local paths is a single row owned by whichever library
scanned it first, so that owner's sweep destroyed a row another library still served —
taking collection membership and playout references with it, irreversibly.

This is #494's deferred "option 2":

- New JellyfinMusicVideo : MusicVideo (ItemId/Etag), mirroring JellyfinMovie — TPT table,
  varchar(36), ItemId index. Dual-provider migration Add_JellyfinMusicVideo.
- New IMediaServerMusicVideoRepository + JellyfinMusicVideoRepository: itemId-keyed
  existing-set/lookup and Flag{Normal,Unavailable,FileNotFound} seams, all scoped per
  library via LibraryPath.LibraryId.
- New MediaServerMusicVideoLibraryScanner base; JellyfinMusicVideoLibraryScanner folds
  onto it and keeps the #177/#488/#497/#500 metadata-reconcile logic verbatim.
- The sweep now soft-trashes (FileNotFound) instead of deleting, so removal is reversible
  and EmptyTrash-governed. DeleteEmptyArtists consequently no longer fires from a sweep.
- Pre-identity rows are ADOPTED in place: the identity row is inserted against the same
  MediaItem id, scoped to the scanned library's own LibraryPath, so collection membership
  survives and a local/second-library row is never hijacked.
- AddMusicVideo normalizes Path/PathHash to the path-REPLACED local path; the projection
  fills them from the server-reported path, which would break every later PathHash lookup.

Docs: scan.musicvideo-reconciliation relocated to docs/decisions/archive/scan.md as
superseded; new active record scan.musicvideo-server-identity.

fixes #496

ErsatzTV.Scanner/Program.cs

@@ -241,6 +241,7 @@ public class Program
                 services.AddScoped<IJellyfinApiClient, JellyfinApiClient>();
                 services.AddScoped<IJellyfinCollectionRepository, JellyfinCollectionRepository>();
                 services.AddScoped<IJellyfinMovieRepository, JellyfinMovieRepository>();
+                services.AddScoped<IJellyfinMusicVideoRepository, JellyfinMusicVideoRepository>();
                 services.AddScoped<IJellyfinTelevisionRepository, JellyfinTelevisionRepository>();
                 services.AddScoped<IJellyfinPathReplacementService, JellyfinPathReplacementService>();