Add back accidentally deleted OPDS routes

Resolves https://github.com/hkalexling/Mango/issues/255#issuecomment-1097588181

src/server.cr

@@ -23,6 +23,7 @@ class Server
     AdminRouter.new
     ReaderRouter.new
     APIRouter.new
+    OPDSRouter.new
 
     {% for path in %w(/api/* /uploads/* /img/*) %}
       options {{path}} do |env|