Add rollout migration tooling and background migration (#37348)

## What changed

- Add `codex migrate-rollouts` with dry-run inspection by default, explicit `--apply`, thread filtering, optional I/O throttling, progress output, and JSON or verbose reports.
- Add the disabled-by-default `background_paginated_rollout_migration` feature to migrate legacy local sessions at startup before rollout compression begins.
- Persist a migration cursor and skipped-file fingerprints so later startups avoid full rescans while retrying changed files and recovering pending migrations.
- Coordinate migration with live writers and emit metrics for manual and startup runs.

## Testing

- Cover startup cursor advancement and lookback, pending migration recovery, live-writer coordination, changed empty rollouts, and progress reporting.

GitOrigin-RevId: 276ac506c50ebec5140fd319faca1bb998172061

codex-rs/app-server/tests/suite/v2/rollout_migration.rs

@@ -83,7 +83,7 @@ async fn migrated_legacy_thread_cold_resume_preserves_model_context() -> Result<
     let report = store
         .migrate_rollouts(RolloutMigrationOptions {
             mode: RolloutMigrationMode::Apply,
-            max_mib_per_second: 1024,
+            max_mib_per_second: Some(1024),
             ..RolloutMigrationOptions::default()
         })
         .await?;