feat: implement article search with PostgreSQL full-text search

- Configure Wagtail database search backend with English search config
- Add django.contrib.postgres to INSTALLED_APPS for full PG FTS support
- Expand ArticlePage.search_fields: body_text (excl. code blocks),
  AutocompleteField(title), RelatedFields(tags), FilterFields
- Add search view at /search/?q= with query guards (strip, max 200 chars,
  empty/whitespace handling) and pagination preserving query param
- Replace nav Subscribe CTA with compact search box (desktop + mobile)
- Add search box to article index page alongside category/tag filters
- Create search results template reusing article_card component
- Add update_index to deploy entrypoint for automated reindexing
- Update existing tests for nav change, add comprehensive search tests

Closes #41

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

deploy/entrypoint.prod.sh

@@ -5,6 +5,7 @@ python manage.py tailwind install --no-input
 python manage.py tailwind build
 python manage.py migrate --noinput
 python manage.py collectstatic --noinput
+python manage.py update_index
 
 # Set Wagtail site hostname from first entry in ALLOWED_HOSTS
 python manage.py shell -c "