fix(email): detect and surface webhook send failures instead of claiming success

send_webhook_email() only returned False on a network exception — a
webhook that responded with a non-2xx status (bad URL, wrong API key,
misconfigured Power Automate flow) was still reported as delivered. Now
checks response.ok and returns False on rejection too.

send_password_setup_email() also never checked the return value at all,
so the admin's 'Send Email' action (including resending a password-reset
email after the user already set their password) always showed success
even when nothing was actually sent. Now raises a clear error instead.

backend/app/services/password_setup_service.py

@@ -65,7 +65,11 @@ def send_password_setup_email(db: Session, user: User) -> None:
         f"{set_password_url}\n\n"
         "This link expires in 24 hours and can only be used once."
     )
-    send_webhook_email(db, to=user.email, subject=subject, message=message, full_name=user.full_name)
+    sent = send_webhook_email(db, to=user.email, subject=subject, message=message, full_name=user.full_name)
+    if not sent:
+        raise BusinessRuleViolation(
+            "Failed to send the email — check the webhook URL/API key in Settings and the server logs."
+        )
 
 
 def _get_valid_token_or_raise(db: Session, token: str) -> PasswordSetupToken: