Speed up unfinished history migration while preserving all messages and
existing processing limits.
Keep migration diagnostics under the configured logging policy and surface
preparation failures.
Fixes#462
Start each account as soon as its message storage is ready, then continue older history updates without delaying normal account availability. Keep already started accounts online when maintenance needs to be retried.
Handle preparation failures, stale completions, early exits, logout, and shutdown consistently. Add lifecycle coverage for startup ordering, automatic retries, cancellation, and account availability.
DrSmoothl