Advanced features: from good to great

Day 3 adds what separates good from great: A/B-tested template variants with consistent hashing, batching with count, time and hybrid flushes, timezone-aware digests, a leader-run scheduler with UTC storage and a best-time-to-send jitter, a multi-channel saga with fallback and escalation, an in-app notification center with an inbox, an unread counter and cross-server WebSocket delivery, and hierarchical, versioned preferences.