SMS marketing: scheduling, admin broadcasts, and true delivery tracking

Scheduled sends — queue an SMS blast for later; campaign-side sends use the campaign time zone.
Per-recipient delivery tracking — every message now records delivered or failed per recipient (previously only failures were logged), with progress updating live while a blast runs.
Admin broadcast composer — staff can send global broadcasts and audit everything in one unified SMS log.
Fixed — Hebrew text no longer corrupts in CSV exports, and team blasts appear correctly in the Individual Messages view.