feat: configure SMTP email notifications across all services

Added comprehensive email configuration to enable notifications for
password resets, user invitations, system alerts, and backup failures.

**Services configured:**

- **Vaultwarden** (vault/compose.yaml):
  - SMTP settings for password resets, 2FA, emergency access emails
  - Uses IONOS SMTP with force_tls security
  - Sender: hi@pivoine.art

- **Linkwarden** (links/compose.yaml):
  - Email server configuration for user invitations
  - Password reset functionality via email
  - Uses smtp:// connection string format

- **n8n** (n8n/compose.yaml):
  - SMTP mode enabled for workflow notifications
  - User invitation emails
  - Password reset support
  - SSL-secured connection

- **Netdata** (netdata/compose.yaml, health_alarm_notify.conf, msmtprc):
  - Health alarm notifications via email
  - MSMTP configuration for sending alerts
  - Notifications sent to valknar@pivoine.art
  - Alerts for system issues, resource exhaustion, service failures

**Common SMTP settings** (from .env):
- Provider: IONOS (smtp.ionos.de:465)
- From address: hi@pivoine.art
- Transport: SMTP with TLS/SSL
- Admin email: valknar@pivoine.art

**Backrest notifications:**
- Configured via web UI at restic.pivoine.art
- Supports webhooks to Gotify for push notifications
- Email notifications can be added through UI settings

All services now have proper email notification capabilities for
improved monitoring, user management, and security features.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

links/compose.yaml

@@ -12,6 +12,8 @@ services:
       NEXTAUTH_URL: https://${LINKS_TRAEFIK_HOST}
       MEILI_ADDR: http://linkwarden_meilisearch:7700
       MEILI_MASTER_KEY: ${LINKS_MEILI_MASTER_KEY}
+      EMAIL_FROM: ${EMAIL_FROM}
+      EMAIL_SERVER: smtp://${EMAIL_SMTP_USER}:${EMAIL_SMTP_PASSWORD}@${EMAIL_SMTP_HOST}:${EMAIL_SMTP_PORT}
     volumes:
       - linkwarden_data:/data/data
     depends_on: