feat: resumable backfill (#10745)

## 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.

codex-rs/core/src/state_db.rs

@@ -31,11 +31,9 @@ pub(crate) async fn init_if_enabled(
     config: &Config,
     otel: Option<&OtelManager>,
 ) -> Option<StateDbHandle> {
-    let state_path = codex_state::state_db_path(config.codex_home.as_path());
     if !config.features.enabled(Feature::Sqlite) {
         return None;
     }
-    let existed = tokio::fs::try_exists(&state_path).await.unwrap_or(false);
     let runtime = match codex_state::StateRuntime::init(
         config.codex_home.clone(),
         config.model_provider_id.clone(),
@@ -55,7 +53,17 @@ pub(crate) async fn init_if_enabled(
             return None;
         }
     };
-    if !existed {
+    let should_backfill = match runtime.get_backfill_state().await {
+        Ok(state) => state.status != codex_state::BackfillStatus::Complete,
+        Err(err) => {
+            warn!(
+                "failed to read backfill state at {}: {err}",
+                config.codex_home.display()
+            );
+            true
+        }
+    };
+    if should_backfill {
         let runtime_for_backfill = Arc::clone(&runtime);
         let config_for_backfill = config.clone();
         let otel_for_backfill = otel.cloned();