feat: add fulltext search with ?q= parameter

Filter posts by case-insensitive match in title and body. Extracted
filter_by_lang and search_posts helpers to keep complexity in check.

CHANGELOG.md

@@ -30,6 +30,16 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - CSRF protection on `POST /admin/preview`.
 - Symlink-safe containment in `Store#media_file` via `File.realpath`.
 
+### Added
+
+- **Scheduled publishing:** a `publish_at` frontmatter field hides posts from the
+  public API and feeds until their publication date arrives. The admin form
+  includes a Publish at date picker.
+- **JSON Feed:** `GET /api/volumen/feed.json` serves a jsonfeed.org v1.1 feed
+  with `content_html` and `date_published` per item.
+- **Fulltext search:** `GET /api/volumen/posts?q=…` filters posts by
+  case-insensitive match in title and body.
+
 ### Changed
 
 - Split `lib/volumen/server.rb` into `api_routes.rb` and `admin_routes.rb` modules,