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.
Add ReleaseMedium enum (Cut/Session/Mix) and two 1:1 satellite entities
(SessionMetadata, MixMetadata) with EF configs and an additive migration.
ReleaseDto.ReleaseType is now nullable, nulled for non-Cut at the converter.
Existing releases default to Cut via column default; no data migration.
daniel-c-harvey