Override ActiveStorage.signed_id_verifier instead of assigning

Guard against the case where ActiveStorage.verifier isn't yet initialized at load time.

Yes, you're not supposed to load AR models in initializers, but it's easy to do accidentally as long as we don't prevent it. We should be resilient against it wherever practical.

activestorage/app/models/active_storage/blob.rb

@@ -35,7 +35,6 @@ class ActiveStorage::Blob < ActiveRecord::Base
   include ActiveStorage::Blob::Representable
 
   self.table_name = "active_storage_blobs"
-  self.signed_id_verifier = ActiveStorage.verifier
 
   MINIMUM_TOKEN_LENGTH = 28
 
@@ -130,9 +129,17 @@ class ActiveStorage::Blob < ActiveRecord::Base
     end
 
     # Customize signed ID purposes for backwards compatibility.
-    def combine_signed_id_purposes(purpose)
+    def combine_signed_id_purposes(purpose) #:nodoc:
       purpose.to_s
     end
+
+    # Customize the default signed ID verifier for backwards compatibility.
+    #
+    # We override the reader (.signed_id_verifier) instead of just calling the writer (.signed_id_verifier=)
+    # to guard against the case where ActiveStorage.verifier isn't yet initialized at load time.
+    def signed_id_verifier #:nodoc:
+      @signed_id_verifier ||= ActiveStorage.verifier
+    end
   end
 
   # Returns a signed ID for this blob that's suitable for reference on the client-side without fear of tampering.