Separate consent per channel (email, SMS, push)

GDPR Art. 7 requires consent to be 'specific' — consent for email marketing does not cover SMS marketing, and a single marketingOptIn boolean that authorizes all channels conflates legally distinct consent decisions. TCPA §227(b)(1)(A) requires 'prior express written consent' specifically for SMS, which is a higher bar than CAN-SPAM email consent. ePrivacy Art. 13 applies channel by channel. Practically: a user who opts into email newsletters may not want SMS promotions — collapsing both into one flag generates TCPA exposure on every marketing SMS sent to users who only checked the email box.

Add per-channel consent columns to the database and expose per-channel controls in the notification preferences UI.

-- Per-channel consent columns (default false per GDPR)
ALTER TABLE users ADD COLUMN email_marketing_opt_in BOOLEAN NOT NULL DEFAULT false;
ALTER TABLE users ADD COLUMN sms_marketing_opt_in   BOOLEAN NOT NULL DEFAULT false;
ALTER TABLE users ADD COLUMN push_marketing_opt_in  BOOLEAN NOT NULL DEFAULT false;

// Notification preferences — granular per channel
<form>
  <label><input type="checkbox" name="emailMarketing" /> Email updates and newsletters</label>
  <label><input type="checkbox" name="smsMarketing" /> SMS promotions (msg & data rates may apply; reply STOP to opt out)</label>
  <label><input type="checkbox" name="pushMarketing" /> In-app promotional notifications</label>
</form>

Every outbound SMS send must check sms_marketing_opt_in, not the shared marketingOptIn column.