Danbooru::Http: add default connection timeout.

* Add a three second connection timeout to all http requests. By default
  http.rb doesn't have any timeouts, so it can hang forever trying to
  connect if there are any network issues.

* Return a fake 522 error in the event of a timeout so that callers
  don't have to deal with TimeoutError exceptions, instead they can treat
  timeouts as normal 5xx errors (which most callers already handle).

app/logical/danbooru/http.rb

@@ -1,5 +1,7 @@
 module Danbooru
   class Http
+    DEFAULT_TIMEOUT = 3
+
     attr_writer :cache, :http
 
     class << self
@@ -42,6 +44,9 @@ module Danbooru
       else
         raw_request(method, url, **options)
       end
+    rescue HTTP::TimeoutError
+      # return a synthetic http error on connection timeouts
+      ::HTTP::Response.new(status: 522, body: "", version: "1.1")
     end
 
     def cached_request(method, url, **options)
@@ -60,7 +65,7 @@ module Danbooru
     end
 
     def http
-      @http ||= ::HTTP.use(:auto_inflate).headers(Danbooru.config.http_headers).headers("Accept-Encoding" => "gzip")
+      @http ||= ::HTTP.timeout(DEFAULT_TIMEOUT).use(:auto_inflate).headers(Danbooru.config.http_headers).headers("Accept-Encoding" => "gzip")
     end
   end
 end