refactor(email): replace Task.WhenAll with foreach in SendEmailBehavior

Changed the SendFinalEmailToReceiversAsync method to use a sequential `foreach` loop
instead of `Task.WhenAll` for sending history commands. Added a null check for
receiver emails to prevent potential exceptions.

EnvelopeGenerator.Finalizer/appsettings.Job.json

@@ -1,8 +1,11 @@
 {
   "FinishEnvelopeJob": {
-    "CronExpression": "* * * * * ?"
+    "CronExpression": "0 0/1 * 1/1 * ? *"
   },
   "EnvelopeTaskApiJob": {
-    "CronExpression": "* * * * * ?"
+    "CronExpression": "0 0/1 * 1/1 * ? *"
+  }
+  "Expressions": {
+    "PerSec": "* * * * * ?"
   }
 }