The ScrobbleViewSet was using lookup_field="uuid" which caused
API detail views to fail for scrobbles with NULL UUIDs. Rather
than backfilling all NULL UUIDs, switch to pk lookup which is
always populated.
Changes:
- Remove lookup_field="uuid" from ScrobbleViewSet (defaults to pk)
- Remove extra_kwargs from ScrobbleSerializer (URL now uses pk)
- Update regenerate_share_token action signature to use **kwargs