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>

ai/compose.yaml

@@ -53,14 +53,12 @@ services:
       RAG_EMBEDDING_MODEL: ${AI_RAG_EMBEDDING_MODEL:-text-embedding-3-small}
       VECTOR_DB: ${AI_VECTOR_DB:-pgvector}
 
-      # Email configuration (IONOS SMTP)
-      SMTP_HOST: ${EMAIL_SMTP_HOST}
-      SMTP_PORT: ${EMAIL_SMTP_PORT}
-      SMTP_USER: ${EMAIL_SMTP_USER}
-      SMTP_PASSWORD: ${EMAIL_SMTP_PASSWORD}
+      # Email configuration (Mailpit SMTP relay)
+      SMTP_HOST: net_mailpit
+      SMTP_PORT: 1025
       SMTP_FROM_EMAIL: ${EMAIL_FROM}
       SMTP_USE_TLS: false
-      SMTP_USE_SSL: true
+      SMTP_USE_SSL: false
 
     volumes:
       - ai_webui_data:/app/backend/data