fix(email): SMTP scheduled-purchase notification goes to NOTIFY_EMAIL instead of the approver #1590

SMTPSender.SendScheduledPurchaseNotification (internal/email/smtp_sender.go, ~line 475) sends the "Purchase Scheduled" email, which includes the purchase's cancel link, to the shared NOTIFY_EMAIL address. #111 moved the other per-purchase emails to the approver or the plan's recipients. This one was missed, and it isn't one of #111's listed items.

Fix: send it to the same recipients as the other per-purchase emails (the approver or the plan's notification list), as the other senders do since #1275. Add a test that asserts the recipient.

Split out of #111 when closing it.