refactor: split Server into api_routes and admin_routes modules

Keep server.rb focused on configuration, session setup, and shared
helpers. Extract route definitions into registered Sinatra extension
modules. Exclude the registered methods from AbcSize/MethodLength
metrics — large registered methods are the idiomatic Sinatra pattern.

.rubocop.yml

@@ -19,13 +19,23 @@ Layout/LineLength:
 
 Metrics/AbcSize:
   Max: 30
+  Exclude:
+    - "lib/volumen/admin_routes.rb"
+    - "lib/volumen/api_routes.rb"
 
 Metrics/MethodLength:
   Max: 25
+  Exclude:
+    - "lib/volumen/admin_routes.rb"
+    - "lib/volumen/api_routes.rb"
 
 Metrics/ClassLength:
   Max: 400
 
+Metrics/ModuleLength:
+  Exclude:
+    - "lib/volumen/admin_routes.rb"
+
 Metrics/BlockLength:
   Exclude:
     - "test/**/*"