Subscriber preference center #54

Background

Unsubscribe is all-or-nothing. Contacts can't pick lists or topics or lower the frequency. There's no preference page anywhere in the API.

Why it matters: Gmail and Yahoo enforce complaint rates under 0.3%, and a preference center gives readers an option other than "report spam". CAN-SPAM and GDPR expect clear subscription controls, and embedding platforms need topic-level opt-outs (product updates vs marketing).

Proposed fix

• Add a hosted, token-authenticated preference page with per-topic/list toggles and "unsubscribe from all".
• Respect topic opt-outs at send time.
• Expose preferences through the API.

Acceptance criteria

• The unsubscribe link offers a manage-preferences page.
• Opting out of one topic stops only that topic's sends.
• "Unsubscribe from all" still works with one click (RFC 8058, see Add RFC 8058 one-click unsubscribe headers to all marketing sends #23).
• Preferences can be read and updated via the API.