Recompute Actual Pause All

Batch backfill across every completed session.

Streams sessions in chronological order. Idempotent — sessions already on the corrected anchor (e.g. ones that ran after deploy and so carry last_word_end_at on their user segments) get the same numbers back out, so no row is rewritten unnecessarily. With dry_run=true, walks the data and reports counts without writing.