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.

test/server_test.rb

@@ -153,4 +153,29 @@ class ServerTest < Minitest::Test
     assert_includes data["items"].first["content_html"], "<strong>world</strong>"
     refute_includes data["items"].map { |i| i["title"] }, "Draft"
   end
+
+  def test_search_by_title
+    get "/api/volumen/posts?q=Hello"
+    data = JSON.parse(last_response.body)
+    assert_equal 1, data["total"]
+    assert_equal "hello", data["posts"].first["slug"]
+  end
+
+  def test_search_by_body
+    get "/api/volumen/posts?q=world"
+    data = JSON.parse(last_response.body)
+    assert_equal 1, data["total"]
+  end
+
+  def test_search_no_match
+    get "/api/volumen/posts?q=nonexistent"
+    data = JSON.parse(last_response.body)
+    assert_equal 0, data["total"]
+  end
+
+  def test_search_is_case_insensitive
+    get "/api/volumen/posts?q=HELLO"
+    data = JSON.parse(last_response.body)
+    assert_equal 1, data["total"]
+  end
 end