Notifications (fire-and-forget, non-blocking) for: review submitted,
approved/rejected/revision requested, task assigned/completed, issue
assigned/status changed, campaign created, user invited. Emails render
in user's preferred language (EN/AR) with RTL support. Adds
preferred_language to Users, syncs from frontend language toggle.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
