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>

e2e/test_home.py

@@ -37,11 +37,10 @@ def test_theme_toggle_adds_dark_class(page: Page, base_url: str) -> None:
 
 
 @pytest.mark.e2e
-def test_nav_subscribe_cta_present(page: Page, base_url: str) -> None:
+def test_nav_search_box_present(page: Page, base_url: str) -> None:
     page.goto(f"{base_url}/", wait_until="networkidle")
     nav = page.locator("nav")
-    # Nav has a Subscribe CTA link (not a form — wireframe spec)
-    expect(nav.get_by_role("link", name="Subscribe")).to_be_visible()
+    expect(nav.locator('input[name="q"]')).to_be_visible()
 
 
 @pytest.mark.e2e