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>

net/msmtprc

@@ -2,19 +2,15 @@
 
 # Set default values for all accounts
 defaults
-auth           on
-tls            on
-tls_trust_file /etc/ssl/certs/ca-certificates.crt
+auth           off
+tls            off
 logfile        /var/log/msmtp.log
 
-# IONOS SMTP account
-account        ionos
-host           smtp.ionos.de
-port           465
-tls_starttls   off
+# Mailpit SMTP relay account
+account        mailpit
+host           net_mailpit
+port           1025
 from           hi@pivoine.art
-user           hi@pivoine.art
-password       jaquoment
 
 # Set default account
-account default : ionos
+account default : mailpit