feat(web-ui): add private-GitHub ingestion via PAT

[filipchristiansen]

Why

Until now the gitingest.com web UI could ingest only public repositories.
Teams often need to analyse private GitHub repos, so we’re adding a way to supply a personal access token (PAT).

What’s inside

Area	Key changes
UI	• New “🔒 Private Repository” checkbox in git_form.jinja that reveals a masked PAT field. • Checkbox state + token are posted with the form.
Server routes	• index.py, dynamic.py, query_processor.py accept token from the form and thread it downstream.
Ingestion stack	• gitingest.entrypoint.parse_query and query_parsing now carry token, letting try_domains_for_user_and_repo() resolve bare user/repo slugs against GitHub also when a PAT is present.
Tests	• Added "token": "" to form_data in tests/test_flow_integration.py to keep path-coverage intact.

Limitation: This PR enables PAT-protected cloning only for GitHub; other hosts (GitLab, Gitea, etc.) remain public-only for now.