test(server): migrate-dev-db refuses pre-V2 sources with legacy threads

Reproduces #19018: a migration-54 source with a projection_threads row
upgrades to an empty V2 projection and the command reports success.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
e0fba36b0dJulius Marminge committed on 10/21/2026, 10:31:46 PMparent70e5e38
1 file changedLine totals unavailable