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>

config/settings/base.py

@@ -29,6 +29,7 @@ INSTALLED_APPS = [
     "django.contrib.messages",
     "django.contrib.staticfiles",
     "django.contrib.sitemaps",
+    "django.contrib.postgres",
     "taggit",
     "modelcluster",
     "wagtail.contrib.forms",
@@ -152,3 +153,10 @@ STORAGES = {
 }
 
 TAILWIND_APP_NAME = "theme"
+
+WAGTAILSEARCH_BACKENDS = {
+    "default": {
+        "BACKEND": "wagtail.search.backends.database",
+        "SEARCH_CONFIG": "english",
+    }
+}