feat: add optional Wagtail CMS integration

[h0pers]

Summary

• Add use_wagtail cookiecutter option (default: n) to optionally integrate Wagtail CMS into generated projects
• Create cms app with HomePage model (Page + RichTextField) and app-level templates (base.html, home_page.html)
• Add wagtail.py settings component with WAGTAIL_SITE_NAME, WAGTAILADMIN_BASE_URL, WAGTAILDOCS_EXTENSIONS, and DATA_UPLOAD_MAX_NUMBER_FIELDS
• Wire up URL routing: /cms/ (Wagtail admin), /documents/ (document serving), / (catch-all page serving)
• Add all 13 required Wagtail apps to INSTALLED_APPS and RedirectMiddleware to MIDDLEWARE
• Extend post-gen hook to remove Wagtail files when use_wagtail=n
• Expand CI matrix to cover all use_drf x use_wagtail combinations (4 jobs)

Test plan

• Generate project with use_wagtail=y use_drf=y — verify all wagtail files present, settings correct, app starts
• Generate project with use_wagtail=y use_drf=n — verify wagtail works without DRF config
• Generate project with use_wagtail=n — verify no cms app, no wagtail settings, no wagtail URLs
• Run docker compose -f docker-compose.debug.yml up --build and confirm /cms/ admin loads
• Run pre-commit on generated project — verify no lint errors
• CI matrix passes all 4 combinations