Create our own sitemap.xml

[jefftriplett]

We need to generate our own sitemap.xml for the new Django site so search engines (and tools like the old-post importer in scripts/fetch_old_posts.py) have an authoritative list of URLs.

Why

• Related to Create redirects from old sitemap #1 (redirects from old sitemap) — once we have our own sitemap we can diff it against the legacy one to confirm every old URL has a redirect target.
• SEO: give crawlers a canonical list of pages on the new site.

Suggested approach

• Use Django's built-in django.contrib.sitemaps framework.
• Add django.contrib.sitemaps to INSTALLED_APPS in config/settings.py.
• Define Sitemap subclasses for each content type we want indexed (blog posts/announcements, event pages, static/flat pages).
• Wire sitemap view into config/urls.py at /sitemap.xml.

Acceptance

• GET /sitemap.xml returns a valid sitemap covering all public pages.
• Sitemap is referenced from robots.txt.