Implements Task 4 Step 2 of the provenance-backfill plan. The live
spike (Step 1) and manual verification (Step 3) were not run in this
environment because a second TDLib client would corrupt the running
worker's authenticated session; the absolute-offset assumption is
noted as pending live verification on deploy.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>