Completes the RunPod backend for testing.

Pods pool (runpod_worker.py):
- RunpodPodPool: provision via _select_gpu (cheaper=lowest $/hr, faster=SECURE+
  throughput, across allowed pools, spot-aware) → create_pod → wait_ready →
  vLLM health-check; least-loaded per-request acquire, provision-on-demand up to
  max_pods; background scaler thread tears down idle pods to min after
  idle_timeout_s, keeps min warm, reaps. get_pod_pool registry + pods_status().
- Budget enforcement at provision: global $/hr rate cap + per-model & global
  cumulative spend caps (rolling period) via the ledger; refuses over budget.

Cost ledger (runpod_ledger.py): persistent per-model + global spend, trailing
rolling windows (hour/day/week/month), totals() for stats. Final pod cost booked
on teardown; live pod cost added on top for enforcement/stats (no double count).

Backend (backends/runpod.py): pods mode wired to the poo...