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/mailpit-relay.yaml

@@ -0,0 +1,11 @@
+# Mailpit SMTP Relay Configuration
+# Relays all outbound emails through IONOS SMTP
+
+# Default relay for all emails
+relay:
+  host: ${EMAIL_SMTP_HOST}
+  port: ${EMAIL_SMTP_PORT}
+  username: ${EMAIL_SMTP_USER}
+  password: ${EMAIL_SMTP_PASSWORD}
+  starttls: true
+  auth: plain