[BE-052] Sweep: Concurrent candidate processing

[ONEONUORA]

Description\nProcess auto-sweep actions concurrently rather than sequentially.\n\n## Files to Edit/Create\n- backend/src/services/sweep_worker.rs\n\n## Acceptance Criteria\n- Spawn tokio tasks to execute sweeps in parallel.\n\n## Guidance / Hints\nUse join_all and limit concurrency rates.