fix(691): revert entity-level null guard, guard read sites instead

The prior commit (a4700185b) made SongMetadata.Artists/AlbumArtists
coalesce null to [] via backing-field getters, reasoning that EF
Core's PreferField access mode never observes the getter. Adversarial
review disproved this on real TvContext/SQLite: a single read of
.Artists on a TRACKED entity mutates the backing field through the
getter, flips the entity to Modified, and the next SaveChanges writes
[] over what was a NULL column -- silent data loss waiting on the
first tracked reader (today all readers happen to be AsNoTracking).

This also reversed docs/decisions/records/api/selection-projection-include-chain.md
(#671) without the doc update CLAUDE.md requires; #691 is that
record's own "sweep by FIELD" follow-up, so it should follow the
record, not contradict it.

Revert SongMetadata.cs to plain auto-properties (byte-identical to
origin/main, BOM still stripped per the #311 gate). Guard the read
sites instead, per the #671 convention (Optional(...).Flatten(),
matching Playouts/Mapper.cs and MediaItems/Mapper.cs):

- SongVideoGenerator.cs: hoist `artists`/`albumArtists` locals once
  near the top of the metadata loop instead of repeating the guard at
  each of the six former call sites.
- MediaCollectionRepository.cs (GroupIntoFakeCollections): guard the
  two AlbumArtists reads at lines ~1147/~1160 that #691 never named --
  dropping the entity-level fix without these would trade one bug for
  two.

Verified RED per guard by removing only the Optional(...).Flatten()
clause (not the whole file): the artists local throws
ArgumentNullException at SongVideoGenerator.cs:88, the albumArtists
local at :89 (List.ToList() on a null IList<string> source -- same
loaded-gun shape the review demonstrated, precise exception type is
ArgumentNullException rather than NullReferenceException since the
throw site is Enumerable.ToList's null-source check). Restored both;
existing SongVideoGeneratorTests still pass. Full ErsatzTV.Core.Tests:
685 passed (1 pre-existing skip), ErsatzTV.Tests: 1996 passed (4
pre-existing skips), 0 failures in each. No EF model drift
(`dotnet ef migrations has-pending-model-changes` reports none).
`dotnet format --verify-no-changes` on the three touched files exits
0.

Refs #691

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ErsatzTV.Core/Domain/Metadata/SongMetadata.cs

@@ -2,30 +2,9 @@ namespace ErsatzTV.Core.Domain;
 
 public class SongMetadata : Metadata
 {
-    // backing fields so EF Core's default PreferField access mode reads/writes the raw
-    // (possibly-null) value during materialization/change-tracking, while every other
-    // caller goes through the property getter and always sees a non-null list. This is
-    // the single guarded boundary for the nullable `Artists`/`AlbumArtists` primitive
-    // collections (ersatztv#691) -- untagged songs persist these columns as NULL
-    // (`FallbackMetadataProvider.GetSongMetadata` never assigns them), so every reader
-    // needs the same `?? []` guard `LibraryBrowseItemMapper` already applied by hand.
-    private IList<string> _artists;
-    private IList<string> _albumArtists;
-
     public string Album { get; set; }
-
-    public IList<string> Artists
-    {
-        get => _artists ??= [];
-        set => _artists = value;
-    }
-
-    public IList<string> AlbumArtists
-    {
-        get => _albumArtists ??= [];
-        set => _albumArtists = value;
-    }
-
+    public IList<string> Artists { get; set; }
+    public IList<string> AlbumArtists { get; set; }
     public string Track { get; set; }
     public string Comment { get; set; }
     public int SongId { get; set; }

ErsatzTV.Core/FFmpeg/SongVideoGenerator.cs

@@ -1,4 +1,4 @@
-using System.Globalization;
+using System.Globalization;
 using System.Text;
 using ErsatzTV.Core.Domain;
 using ErsatzTV.Core.Interfaces.FFmpeg;
@@ -85,6 +85,9 @@ public class SongVideoGenerator : ISongVideoGenerator
 
             var sb = new StringBuilder();
 
+            List<string> artists = Optional(metadata.Artists).Flatten().ToList();
+            List<string> albumArtists = Optional(metadata.AlbumArtists).Flatten().ToList();
+
             if (detailsStyle)
             {
                 if (!string.IsNullOrWhiteSpace(metadata.Title))
@@ -92,17 +95,17 @@ public class SongVideoGenerator : ISongVideoGenerator
                     sb.Append(CultureInfo.InvariantCulture, $"{{\\fs{largeFontSize}}}{metadata.Title}");
                 }
 
-                if (metadata.Artists.Count > 0)
+                if (artists.Count > 0)
                 {
-                    var allArtists = string.Join(", ", metadata.Artists);
+                    var allArtists = string.Join(", ", artists);
                     sb.Append(CultureInfo.InvariantCulture, $"\\N{{\\fs{fontSize}}}{allArtists}");
                 }
             }
             else
             {
-                if (metadata.Artists.Count > 0)
+                if (artists.Count > 0)
                 {
-                    var allArtists = string.Join(", ", metadata.Artists);
+                    var allArtists = string.Join(", ", artists);
                     sb.Append(allArtists);
                 }
 
@@ -111,11 +114,11 @@ public class SongVideoGenerator : ISongVideoGenerator
                     sb.Append(CultureInfo.InvariantCulture, $"\\N\"{metadata.Title}\"");
                 }
 
-                if (metadata.AlbumArtists.Count > 0)
+                if (albumArtists.Count > 0)
                 {
                     var allAlbumArtists = string.Join(
                         ", ",
-                        metadata.AlbumArtists.Filter(aa => !metadata.Artists.Contains(aa)));
+                        albumArtists.Filter(aa => !artists.Contains(aa)));
                     sb.Append(CultureInfo.InvariantCulture, $"\\N{allAlbumArtists}");
                 }
 

ErsatzTV.Infrastructure/Data/Repositories/MediaCollectionRepository.cs

@@ -1144,7 +1144,7 @@ public class MediaCollectionRepository : IMediaCollectionRepository
 
         var allArtists = items.OfType<Song>()
             .SelectMany(s => s.SongMetadata)
-            .Map(sm => sm.AlbumArtists.HeadOrNone().Match(aa => aa, string.Empty))
+            .Map(sm => Optional(sm.AlbumArtists).Flatten().HeadOrNone().Match(aa => aa, string.Empty))
             .Distinct()
             .ToList();
 
@@ -1157,7 +1157,7 @@ public class MediaCollectionRepository : IMediaCollectionRepository
         foreach (Song song in items.OfType<Song>())
         {
             string firstArtist = song.SongMetadata
-                .SelectMany(sm => sm.AlbumArtists)
+                .SelectMany(sm => Optional(sm.AlbumArtists).Flatten())
                 .HeadOrNone()
                 .Match(aa => aa, string.Empty);