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>

templates/blog/article_index_page.html

@@ -23,12 +23,20 @@
     <p class="text-zinc-600 dark:text-zinc-400 mb-6">{{ active_category.description }}</p>
   {% endif %}
 
-  <!-- Category Filters -->
-  <div class="flex flex-wrap gap-3 mb-4">
+  <!-- Filters / Search -->
+  <div class="flex flex-col md:flex-row justify-between gap-6 mb-4">
+    <!-- Category Filters -->
+    <div class="flex flex-wrap gap-3">
     <a href="/articles/{% if active_tag %}?tag={{ active_tag }}{% endif %}" class="px-4 py-2 font-mono text-sm font-bold border transition-colors {% if not active_category %}bg-brand-dark text-brand-light dark:bg-brand-light dark:text-brand-dark border-transparent{% else %}bg-transparent text-zinc-600 dark:text-zinc-400 border-zinc-300 dark:border-zinc-700 hover:text-brand-dark dark:hover:text-brand-light hover:border-brand-dark dark:hover:border-brand-light{% endif %}" {% if not active_category %}aria-current="page"{% endif %}>Categories</a>
     {% for category_link in category_links %}
       <a href="{{ category_link.url }}{% if active_tag %}?tag={{ active_tag }}{% endif %}" class="px-4 py-2 font-mono text-sm font-bold border transition-colors {% if active_category and active_category.slug == category_link.category.slug %}bg-brand-dark text-brand-light dark:bg-brand-light dark:text-brand-dark border-transparent{% else %}bg-transparent text-zinc-600 dark:text-zinc-400 border-zinc-300 dark:border-zinc-700 hover:text-brand-dark dark:hover:text-brand-light hover:border-brand-dark dark:hover:border-brand-light{% endif %}" {% if active_category and active_category.slug == category_link.category.slug %}aria-current="page"{% endif %}>{{ category_link.category.name }}</a>
     {% endfor %}
+    </div>
+    <form action="{% url 'search' %}" method="get" role="search" class="relative w-full md:w-64">
+      <svg class="absolute left-3 top-1/2 -translate-y-1/2 w-4 h-4 text-zinc-400" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="1.5" stroke="currentColor"><path stroke-linecap="round" stroke-linejoin="round" d="m21 21-5.197-5.197m0 0A7.5 7.5 0 1 0 5.196 5.196a7.5 7.5 0 0 0 10.607 10.607Z" /></svg>
+      <input type="search" name="q" placeholder="Search articles..." aria-label="Search articles"
+        class="w-full bg-brand-surfaceLight dark:bg-brand-surfaceDark border border-zinc-300 dark:border-zinc-700 pl-10 pr-4 py-2 font-mono text-sm focus:outline-none focus:border-brand-cyan dark:focus:border-brand-cyan focus:ring-1 focus:ring-brand-cyan transition-shadow" />
+    </form>
   </div>
 
   <!-- Tag Filters -->