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/campaign_crud_service.py

@@ -755,6 +755,15 @@ def complete_campaign(db: Session, campaign_id: str) -> Campaign:
     campaign.completed_at = datetime.utcnow()
     # Flush changes to DB without committing the transaction
     db.flush()
+
+    from app.services.notification_service import notify_user_by_email
+    notify_user_by_email(
+        db, campaign.created_by,
+        preference_key="email_on_campaign_completed",
+        subject=f"Campaign Completed: {campaign.name}",
+        message=f'Your campaign "{campaign.name}" has been completed.',
+    )
+
     # Return campaign
     return campaign