feat: add headless crawl mode for cron/automation usage

[MiquelGomezCorral]

What

Adds a --crawl URL flag that runs a crawl without starting the web server, then exits. No browser, no Flask, no HTTP round-trips needed.

Also adds:

• --port / -p — configure port without env var
• --no-browser — suppress the auto-open browser tab on normal server start

Why

Makes it possible to drive LibreCrawl from a cron job or CI script without running a persistent web server or scripting HTTP API calls:

# cron: crawl nightly, save JSON report
0 2 * * * python main.py --crawl https://example.com --output /reports/nightly.json

# pipe to jq
python main.py --crawl https://example.com | jq '[.data[] | select(.status_code >= 400)]'

# CSV
python main.py --crawl https://example.com --crawl-format csv --output report.csv

Progress is printed to stdout; results go to --output file or stdout if omitted. Exit code is 0 on success, 1 on error.

Changes

• main.py only — one file, no new dependencies
• Reuses the existing WebCrawler, SettingsManager, and export helpers already in the codebase
• Normal server mode is completely unaffected