Configure per-brand SMTP

So application confirmations, sync error emails etc. go out under your own brand, configure SMTP per brand:

1. Admin → Brands → pick a brand.
2. SMTP section: host, port (typically 465 for SSL/TLS or 587 for STARTTLS), user, password.
3. From address + from name — what shows in the recipient's inbox.
4. Save. Password is encrypted with AES-256-CTR; only the brand ID is in plain text.

BAconn caches the transporter per brand ID so we don't open a new connection on every send. On brand update, cache is invalidated automatically.