feat: enforce configurable minimum cron intervals

[Linxiushen]

Summary

• add AUTOMATION_MIN_CRON_INTERVAL_SECONDS with a disabled-by-default value of 0
• reject cron triggers whose shortest sampled interval is below the configured floor
• advertise the effective floor as the maximum of polling, scheduler, and deployment limits
• cover defaults, environment loading, validation boundaries, and capability reporting

Testing

• python -m pytest tests/test_schemas.py tests/test_config.py -q (63 passed)
• ruff check and ruff format --check on changed files
• pycodestyle and pyright on changed files
• SQLite-backed /capabilities endpoint smoke test

Closes #217