feat: add Mailpit SMTP relay and migrate all services

- Add Mailpit service to NET stack with web UI at mailpit.pivoine.art
- Configure Mailpit to relay all emails through IONOS SMTP
- Migrate all 11+ services to use Mailpit instead of direct IONOS SMTP:
  * SEXY: Directus API
  * UTIL: Joplin, Mattermost, Vaultwarden, Tandoor, Linkwarden
  * DEV: Gitea, n8n, Asciinema
  * AI: Open WebUI
  * NET: Netdata (via msmtp)
- Centralize SMTP credentials in mailpit-relay.yaml
- Simplify service configs (no auth/TLS for internal SMTP)
- Enable email monitoring via Mailpit web UI with Basic Auth

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

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

arty.yml

@@ -122,6 +122,9 @@ envs:
     NET_TRACK_DOCKER_IMAGE: ghcr.io/umami-software/umami:postgresql-latest
     NET_TRACK_TRAEFIK_HOST: umami.pivoine.art
     NET_TRACK_DB_NAME: umami
+    # Mailpit SMTP Relay
+    NET_MAILPIT_IMAGE: axllent/mailpit:latest
+    NET_MAILPIT_TRAEFIK_HOST: mailpit.pivoine.art
     # AI Stack
     AI_TRAEFIK_ENABLED: true
     AI_COMPOSE_PROJECT_NAME: ai