Schedule enterprise audit-log exports for self-hosted operators

## Summary

On-demand export exists (`GET /api/v1/orgs/{id}/audit-logs/export`). Roadmap v1.2 wants scheduled export. Self-hosted pilots still copy CSVs by hand.

## Context

- `tests/integration/test_audit_export.py`
- `src/voxforge/api/v1/orgs.py`
- `docs/ROADMAP.md`

## Scope

Start small: cron-friendly script + docs (org-scoped, existing CSV/JSON), not a SaaS email product.

## Acceptance criteria

- [ ] `scripts/audit_export_cron.sh` (or equivalent) documented for VPS cron
- [ ] Uses existing export API + operator token/API key
- [ ] RBAC respected (cannot export another org)
- [ ] Integration or script test for payload shape
- [ ] No object-storage vendor required for v1

Start with src/voxforge/api/v1/orgs.py and tests/integration/test_audit_export.py to understand the existing export endpoint, payload formats, and organization access checks. Then review docs/ROADMAP.md and document a VPS cron-friendly script using an operator token or API key; done means the script, documentation, and a payload-shape or integration test cover CSV/JSON export without requiring object storage.

Written by the indexing model from the issue text.