FindOrCreateRelease now returns (ReleaseDto, bool WasCreated); the CREATE path in UploadAsync
rejects WasCreated=false as a duplicate rather than silently attaching on a lost race.
New nullable Description column (max 4000) on ReleaseEntity, rides the Genre write channel through upload + edit; multiline CMS input. Migration authored, not applied.
Promote the Session/Mix single-track rule from a CMS-form convention to a
domain invariant: declare cardinality as data in MediumRules, enforce it in
UnifiedTrackService before the vault write (no orphan), return 409, and read
the same rule in the batch-form collapse.
daniel-c-harvey