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>

sexy/compose.yaml

@@ -33,12 +33,10 @@ services:
       EXTENSIONS_PATH: ${SEXY_EXTENSIONS_PATH:-./extensions}
       EXTENSIONS_AUTO_RELOAD: ${SEXY_EXTENSIONS_AUTO_RELOAD:-false}
       CONTENT_SECURITY_POLICY_DIRECTIVES__FRAME_SRC: ${SEXY_CONTENT_SECURITY_POLICY_DIRECTIVES__FRAME_SRC}
-      EMAIL_TRANSPORT: ${EMAIL_TRANSPORT}
+      EMAIL_TRANSPORT: smtp
       EMAIL_FROM: ${EMAIL_FROM}
-      EMAIL_SMTP_HOST: ${EMAIL_SMTP_HOST}
-      EMAIL_SMTP_PORT: ${EMAIL_SMTP_PORT}
-      EMAIL_SMTP_USER: ${EMAIL_SMTP_USER}
-      EMAIL_SMTP_PASSWORD: ${EMAIL_SMTP_PASSWORD}
+      EMAIL_SMTP_HOST: net_mailpit
+      EMAIL_SMTP_PORT: 1025
       USER_REGISTER_URL_ALLOW_LIST: ${SEXY_USER_REGISTER_URL_ALLOW_LIST}
       PASSWORD_RESET_URL_ALLOW_LIST: ${SEXY_PASSWORD_RESET_URL_ALLOW_LIST}
     labels: