Archive webmentions if they have been removed from their source

Gemfile.lock

@@ -63,6 +63,7 @@ GEM
       sshkit (~> 1.3)
     capistrano-systemd-multiservice (0.1.0.beta13)
       capistrano (~> 3.7)
+    chronic (0.10.2)
     coderay (1.1.3)
     concurrent-ruby (1.2.3)
     connection_pool (2.4.1)
@@ -482,6 +483,8 @@ GEM
       http (~> 5.0)
       indieweb-endpoints (~> 8.0)
       nokogiri (>= 1.13)
+    whenever (1.0.0)
+      chronic (>= 0.6.3)
     xml-simple (1.1.9)
       rexml
     xrpc (0.1.7.2)
@@ -561,6 +564,7 @@ DEPENDENCIES
   timecop
   warning
   webmention
+  whenever
 
 RUBY VERSION
    ruby 3.3.0p0