core: background job queue for chain submission and long-running work

[Codeinx]

Problem

Submitting a Soroban transaction and waiting for confirmation takes several seconds. Doing that inside a request handler ties up a connection, times out under load, and loses the work entirely if the process restarts mid-flight.

What this adds

New module: src/queue/

• A durable job queue (JobQueue + JobStore) backed by a JSON snapshot file, written with write-to-temp-then-rename on every mutation so a crash mid-write never leaves a half-written file behind. Chosen over BullMQ/Redis to keep this repo's dependency footprint and CI/test setup unchanged — the acceptance criteria call for jobs to survive a restart, not for a specific backend.
• Designed for jobs like transaction submission/confirmation polling, export generation, indexer catch-up, and webhook fan-out (handler registration is by job type, so callers plug in their own).
• Idempotent by construction: a job id that the store has already observed complete is never re-dispatched, including across a simulated crash-and-restart against the same durable file — a job may run twice after a crash, and for a transaction submission that's a real financial event, not a free retry.
• Retry with bounded, jittered exponential backoff, moving to a queryable dead-letter status once maxAttempts is exhausted (deadLetterQueue()).
• metrics() exposes queue depth, oldest-pending age, dead-letter count, and failure rate.

Testing

src/queue/queue.test.ts covers:

• a handler invoked twice via run() only executes once
• the double-run-after-crash case explicitly: two separate JobQueue instances against the same durable file, verifying the handler does not re-run on the second instance
• retry-until-dead-letter, with the last error preserved
• metrics reflecting depth, dead-letter count, and failure rate

npm run lint, npm run build, npm test, and npm audit --audit-level=high are all clean on this branch.

Scope note

This PR only adds src/queue/** (new files) — it does not touch package.json, src/routes/v1/index.ts, or any other file shared with the other two PRs open against this issue tracker (#44, #58), so it should apply cleanly regardless of merge order.

Closes #54.

[joelpeace48-cell]

@Codeinx Nice job