Start an email campaign

Starts the campaign (status RUNNING). Requires an active subscription and passes start checks: at least one provider connected, a schedule, at least one step, at least one recipient, dailyContact >= 2, and at most 30000 recipients (higher for beta accounts). Validation failures return a stable error code (e.g. EMAIL_PROVIDER_MISSING, NO_RECIPIENTS, TOO_MANY_RECIPIENTS, DAILY_CONTACT_TOO_LOW). New integrations should use the multichannel /advanced campaign endpoints instead.