feat: pluggable backends (Redis / SQLite) for cross-host idempotency

[samuelagents]

Motivation

The default store is filesystem-backed, giving crash-safe at-most-once semantics on a single host. Operators running the same job from multiple hosts/containers (HA schedulers, k8s CronJobs with >1 replica) need a shared marker store so the effect is deduplicated across hosts.

Proposal

Extract a Backend interface so the lock/marker store is pluggable, and ship adapters:

• RedisBackend — SET key val NX PX <lease> for the lock; marker in a hash.
• SqliteBackend — single-table marker store with a transaction-guarded acquire.

interface Backend {
  readMarker(key: string): Promise<Marker | null>;
  writeMarker(key: string, marker: Marker): Promise<void>;
  acquireLock(key: string, leaseMs: number): Promise<boolean>;
  releaseLock(key: string): Promise<void>;
}
createOnceStore({ backend: new RedisBackend(client) });

Acceptance

• Backend interface extracted; current fs logic becomes the default FsBackend.
• Redis + SQLite adapters run the same parametrized test suite.
• Two-process concurrency test passes against a shared backend.
• Adapters are optional peer deps — core stays zero-dependency.