The core notification flow, end to end

Day 2 builds the happy path: an idempotent, validated Send API, the transactional outbox and its processor (skip locked, ack before mark, stop on failure), the router's preference, opt-out, quiet-hours and availability rules, templates with caching and locale fallback, one provider interface with retry and fallback flags, the channel worker loop with backoff and DLQ, multi-device push, the status state machine, and verified, deduplicated webhooks.