Artist, ExternalId, OriginalPictureName and OriginalAudioName were
non-nullable with '' sentinels, which forced the unique (Source,
ExternalId) index to rely on a filtered "<> ''" clause. Make them
truly nullable and drop the empty-string sentinels, backfilling
existing rows to NULL. Also fixes a formatting regression
(`track != null&&`) introduced alongside the nullability change.
- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore
Build from backend/Musify.slnx: 0 warnings, 0 errors.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>