feat(email): generic webhook for all notification emails, hide SMTP UI

- Renamed the password-setup webhook (email_webhook.*, /system/email-webhook-config)
  and made it the single transport for all notification emails.
- New webhook_email_service.py shared by password_setup_service and
  notification_service.
- Wired test validated/rejected, campaign completed, and new-MITRE-technique
  notifications through the webhook (previously dead SMTP code, never triggered).
- Added POST /system/email-webhook-test to send a real test email.
- Hid the Email/SMTP settings tab from the UI (SMTP code kept intact, unused).
- Redact email_webhook.api_key on config export.

backend/app/services/mitre_sync_service.py

@@ -391,5 +391,18 @@ def sync_mitre(db: Session) -> dict:
     # Commit all pending changes to the database
     db.commit()
 
+    if created > 0:
+        from app.services.notification_service import notify_all_users_by_email
+
+        notify_all_users_by_email(
+            db,
+            preference_key="email_on_new_mitre_techniques",
+            subject=f"MITRE ATT&CK Updated: {created} new techniques",
+            message=(
+                f"{created} new techniques were added and {updated} were "
+                "updated in the MITRE ATT&CK catalog."
+            ),
+        )
+
     # Return summary
     return summary