The legacy guard returned early once the V2 tables existed, but the server
imports V1 threads at boot rather than in a migration. A source migrated to
V2 without ever booting kept its legacy threads only in projection_threads,
so the copy succeeded with zero threads. Count V1 threads without a V2
thread.created event, as LegacyV1ThreadImporter.pendingThreadCount does,
regardless of schema version. The error now points at the statev2.sqlite a
server leaves behind after importing a copy.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>