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.

lib/volumen/server.rb

@@ -101,10 +101,13 @@ module Volumen
       page = positive_int(params["page"], 1)
       limit = positive_int(params["limit"], 20).clamp(1, 100)
       offset = (page - 1) * limit
+      total = posts.length
       {
         "page" => page,
         "page_size" => limit,
-        "total" => posts.length,
+        "total" => total,
+        "has_next" => offset + limit < total,
+        "has_prev" => page > 1,
         "posts" => posts[offset, limit].to_a.map(&:summary)
       }
     end