FIX: Change default for IncomingEmail#created_via to 0 (unknown) and make NOT NULL (#11782)

Follow up to https://review.discourse.org/t/dev-add-created-via-column-to-incomingemail-pr-11751/18366/6

spec/fabricators/incoming_email_fabricator.rb

@@ -6,6 +6,7 @@ Fabricator(:incoming_email) do
   from_address "foo@example.com"
   to_addresses "someone@else.com"
   imap_sync false
+  created_via 0
 
   raw <<~RAW
     Return-Path: <foo@example.com>