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/models.py

@@ -235,8 +235,27 @@ class ArticlePage(SeoMixin, Page):
 
     search_fields = Page.search_fields + [
         index.SearchField("summary"),
+        index.SearchField("body_text", es_extra={"analyzer": "english"}),
+        index.AutocompleteField("title"),
+        index.RelatedFields("tags", [
+            index.SearchField("name"),
+        ]),
+        index.FilterField("category"),
+        index.FilterField("published_date"),
     ]
 
+    @property
+    def body_text(self) -> str:
+        """Extract prose text from body StreamField, excluding code blocks."""
+        parts: list[str] = []
+        for block in self.body:
+            if block.block_type == "code":
+                continue
+            value = block.value
+            text = value.source if hasattr(value, "source") else str(value)
+            parts.append(text)
+        return " ".join(parts)
+
     def save(self, *args: Any, **kwargs: Any) -> None:
         if not self.category_id:
             self.category, _ = Category.objects.get_or_create(