Add management command to export a site to Hugo

[Copilot]

Export Common Content sites to Hugo-compatible directory structures with YAML front matter, page bundles, media resources, and static assets. Supports incremental export with skip/overwrite/remove modes.

src/commoncontent/actions.py

• export_page(instance, content_dir, mode, force) — exports any BasePage subclass as a Hugo page bundle with YAML front matter, body HTML, rich description, share image, image collections, attachments, and inline <img> references
• export_site(site, outdir, mode, force) — breadth-first export: HomePage → Page → Section → Article, plus static/$domain/ → assets/
• Media files get companion .yaml metadata files (title, mime_type, upload_date, tags; images add alt_text, width, height)
• Three modes: skip (default, no overwrites), overwrite (mtime-aware unless force=True), remove (nuke and rebuild)

src/commoncontent/management/commands/export_hugo.py

• export_hugo <site> <outdir> [--skip|--overwrite|--remove] [--force]
• Site resolved by ID or domain; --remove requires interactive confirmation

Front matter mapping

title: instance.title
subtitle: instance.subtitle
seo_title: instance.seo_title
description: instance.seo_description
draft: true  # unless status == USABLE
publishdate: instance.date_published
lastmod: instance.date_modified
dateCreated: instance.date_created
expirydate: instance.expires
copyright: instance.copyright_notice
icon_name: instance.icon_name
cover: instance.share_image.url

HomePage/Section → _index.html; Page/Article → index.html. Bundle path derived from get_absolute_url().

Tests

26 test cases in tests/test_export_hugo.py using real DB fixtures and tiny test files (10×10 JPEG, text attachment, domain static CSS). Covers all page types, front matter fields, media/attachment metadata, skip/overwrite semantics, series URL paths, and static file export via @override_settings(STATIC_ROOT=...).