Generalize YouTube-backed endpoints behind a source parameter

Removes the last YouTube-specific routes (/youtube/*) and merges them
into the general track/album/playlist endpoints, keyed by a `source`
string validated against TrackSource. This mirrors the earlier track
search merge for albums and playlist-track additions:

- GetAlbums now merges local albums with live YouTube Music search
  results, paginated via a youtubeContinuationToken, deduped against
  already-materialized ExternalAlbum rows.
- New GET /albums/external/{source}/{externalId} and
  GET /tracks/external/{source}/{externalId}/stream replace the
  YouTube-only equivalents.
- POST /playlists/{id}/tracks replaces the separate
  .../youtube-tracks endpoint, accepting an optional Source/ExternalId
  pair alongside TrackId.
- AlbumSearchItemResponse/AlbumsSearchResponse follow the same
  discriminated-union + trailing-HasNextPage-field pattern already
  used for track search, so HasNextPage stays a required field in the
  generated OpenAPI schema.
- Frontend routes renamed accordingly (/albums/youtube/[albumId] ->
  /albums/external/[source]/[externalId], etc.) with a single
  /api/thumbnail proxy replacing /api/youtube/thumbnail.

Musify.Api.Endpoints.YouTubeEndpoints and the standalone
SearchYouTubeAlbums query are removed as a result.

backend/Hosts/Musify.Api/Validators/PlayLists/AddPlayListTrackRequestValidator.cs

@@ -0,0 +1,27 @@
+using FluentValidation;
+using Musify.Api.DataTransferObjects.PlayLists;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Api.Validators.PlayLists;
+
+public sealed class AddPlayListTrackRequestValidator : AbstractValidator<AddPlayListTrackRequest>
+{
+    public AddPlayListTrackRequestValidator()
+    {
+        RuleFor(x => x)
+            .Must(x => x.TrackId.HasValue ^ (x.Source.HasValue && !string.IsNullOrEmpty(x.ExternalId)))
+            .WithMessage("Provide either trackId, or source and externalId, but not both.");
+
+        When(x => x.Source.HasValue, () =>
+        {
+            RuleFor(x => x.Source)
+                .Must(source => source == TrackSource.YouTube)
+                .WithMessage("Unsupported source.");
+
+            RuleFor(x => x.ExternalId)
+                .NotEmpty()
+                .Matches("^[A-Za-z0-9_-]{11}$")
+                .WithMessage("externalId must be a valid YouTube video id.");
+        });
+    }
+}

backend/Hosts/Musify.Api/Validators/PlayLists/AddYouTubeTrackRequestValidator.cs

@@ -1,14 +0,0 @@
-using FluentValidation;
-using Musify.Api.DataTransferObjects.PlayLists;
-
-namespace Musify.Api.Validators.PlayLists;
-
-public sealed class AddYouTubeTrackRequestValidator : AbstractValidator<AddYouTubeTrackRequest>
-{
-    public AddYouTubeTrackRequestValidator()
-    {
-        RuleFor(x => x.VideoId)
-            .NotEmpty()
-            .Matches("^[A-Za-z0-9_-]{11}$");
-    }
-}