Custom tracking domain and custom return-path (bounce) domain per team #50

Background

Tracking, click and unsubscribe links are always built from SendLit's host (utils/mail.ts:5-19, getSiteUrl()). There's no setting for a custom return-path (bounce / MAIL FROM) domain.

Why it matters:

• The Gmail/Yahoo 2024 sender rules require DMARC alignment, and a return-path on the sender's domain gives SPF alignment.
• A shared tracking domain shares link reputation across every tenant, which is a common cause of blocklisting.
• Embedding platforms need both to white-label sends.

Proposed fix

• Add a per-team tracking domain: a CNAME to the API, verified by a DNS check, with TLS. Use it for pixel, click and unsubscribe URLs.
• Add a per-team return-path domain where the ESP supports it (SES custom MAIL FROM, Mailgun/SendGrid/Postmark), with DNS docs.

Related: #4.

Acceptance criteria

• A team with a verified tracking domain gets links on that domain.
• Teams without one fall back to the current host.
• The return-path is applied and documented for supported ESPs.