## Summary
This PR makes SQLite rollout backfill resumable and repeatable instead
of one-shot-on-db-create.
## What changed
- Added a persisted backfill state table:
- state/migrations/0008_backfill_state.sql
- Tracks status (pending|running|complete), last_watermark, and
last_success_at.
- Added backfill state model/types in codex-state:
- BackfillState, BackfillStatus (state/src/model/backfill_state.rs)
- Added runtime APIs to manage backfill lifecycle/progress:
- get_backfill_state
- mark_backfill_running
- checkpoint_backfill
- mark_backfill_complete
- Updated core startup behavior:
- Backfill now runs whenever state is not Complete (not only when DB
file is newly created).
- Reworked backfill execution:
- Collect rollout files, derive deterministic watermark per path, sort,
resume from last_watermark.
- Process in batches (BACKFILL_BATCH_SIZE = 200), checkpoint after each
batch.
- Mark complete with last_success_at at the end.
## Why
Previous behavior could leave users permanently partially backfilled if
the process exited during initial async backfill. This change allows
safe continuation across restarts and avoids restarting from scratch.
