Fix PostgreSQL clone fixture user ownership

[stef-k]

Summary

• attach the already-persisted API user to the clone fixture context before creating its required token navigation
• prevent EF Core from treating that detached user as a new AspNetUsers row
• preserve the complete guarded PostgreSQL clone, rollback, and cleanup assertions

Closes #495.

Validation

• immutable red checkpoint: duplicate PK_AspNetUsers failure reproduced
• corrected guarded PostgreSQL clone workflow: 1 passed, 0 failed/skipped
• PostgreSQL environment/residue evidence: 1 passed, 0 failed/skipped
• related TripClone selection: 4 passed, 0 failed/skipped
• Release test/application build: succeeded, 0 warnings/errors
• LOC policy: passed; pre-existing warnings only
• git diff --check: passed

Database

No migration or production database change. Test-fixture ownership correction only.