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>

apps/blog/tests/test_views.py

@@ -69,8 +69,9 @@ def test_newsletter_forms_render_in_nav_and_footer(client, home_page):
     resp = client.get("/")
     html = resp.content.decode()
     assert resp.status_code == 200
-    # Nav has a Subscribe CTA link (no inline form — wireframe spec)
-    assert 'href="#newsletter"' in html
+    # Nav has a search form instead of Subscribe CTA
+    assert 'role="search"' in html
+    assert 'name="q"' in html
     # Footer has Connect section with social/RSS links (no newsletter form)
     assert "Connect" in html
     assert 'name="source" value="nav"' not in html