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.

lib/volumen/server.rb

@@ -102,14 +102,29 @@ module Volumen
 
     def filtered_posts
       posts = published_posts
-      if params["lang"]
-        lang = params["lang"]
-        posts = posts.select { |post| post.lang == lang || post.all_langs? }
-      end
+      posts = filter_by_lang(posts)
       posts = posts.select { |post| post.tags.include?(params["tag"]) } if params["tag"]
+      posts = search_posts(posts) if params["q"]
       posts
     end
 
+    def filter_by_lang(posts)
+      return posts unless params["lang"]
+
+      lang = params["lang"]
+      posts.select { |post| post.lang == lang || post.all_langs? }
+    end
+
+    def search_posts(posts)
+      query = params["q"].to_s.strip.downcase
+      return posts if query.empty?
+
+      posts.select do |post|
+        post.title.to_s.downcase.include?(query) ||
+          post.body.downcase.include?(query)
+      end
+    end
+
     def tag_counts
       counts = Hash.new(0)
       published_posts.each { |post| post.tags.each { |tag| counts[tag] += 1 } }