Add MailFlow template

[Ahson-Shaikh]

Summary

• Adds the MailFlow template — a self-hosted unified webmail client that connects to your existing IMAP/SMTP accounts.
• Four services: frontend (nginx), backend, Postgres, and Redis. The bundled nginx config hardcodes the backend hostname as backend:3000, so it's replaced with a template-generated config pointing at the actual Easypanel service hostname.
• The backend's APP_URL uses the same cross-service domain pattern as other multi-service templates in this repo ($(PRIMARY_DOMAIN) only resolves for the service that owns the domain, and the backend doesn't have one).

Test plan

• Deployed via the actual one-click-deploy path (createFromSchema) on a live Easypanel instance
• Confirmed DB migrations applied, Redis connected, and both services started cleanly with no crash-looping
• Verified the frontend actually proxies through to the backend (200 on / and /api/health)
• npm run build-templates, npx eslint, npx prettier --check all clean in an isolated worktree

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