feat: add has_next and has_prev to paginated posts response

Clients no longer need to compute page boundaries themselves; the API
now returns boolean flags indicating whether more pages exist in each
direction. Includes tests for first, middle, and last page.

test/server_test.rb

@@ -274,4 +274,31 @@ class ServerTest < Minitest::Test
     data = JSON.parse(last_response.body)
     assert_equal 1, data["total"]
   end
+
+  def test_pagination_has_next_and_prev
+    # Create enough posts to paginate
+    5.times do |i|
+      File.write(File.join(@dir, "post-#{i}.md"), <<~POST)
+        +++
+        title = "Post #{i}"
+        +++
+
+        Body #{i}.
+      POST
+    end
+    get "/api/volumen/posts?page=1&limit=2"
+    data = JSON.parse(last_response.body)
+    assert_equal true, data["has_next"]
+    assert_equal false, data["has_prev"]
+
+    get "/api/volumen/posts?page=2&limit=2"
+    data = JSON.parse(last_response.body)
+    assert_equal true, data["has_next"]
+    assert_equal true, data["has_prev"]
+
+    get "/api/volumen/posts?page=3&limit=2"
+    data = JSON.parse(last_response.body)
+    assert_equal false, data["has_next"]
+    assert_equal true, data["has_prev"]
+  end
 end