fix(server): migrate-dev-db refuses unimported legacy threads#23159

Merge remote-tracking branch 'origin/main' into fix/issue-19018

fix(server): migrate-dev-db checks for legacy threads in the copy's own snapshot

test(server): migrate-dev-db refuses a legacy thread committed after the guard

perf(server): migrate-dev-db legacy guard uses the V2 created-threads index

fix(server): migrate-dev-db refuses V2 sources with unimported legacy threads

Merge remote-tracking branch 'origin/main' into fix/issue-19018

fix(server): migrate-dev-db rejects legacy sources before touching the destination

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

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