Backend: Add routes-f async job enqueue endpoint

## Summary
Add a Routes-F async job enqueue endpoint stub.

## Requirements
- `POST /api/routes-f/jobs` accepts `{ type, payload }`.
- Returns job id and status `queued`.
- No real queue integration required yet.

## Notes
- Use in-memory queue for now.

## Acceptance Criteria
- Invalid job type returns `400`.
- Tests cover job enqueue response.
