Build a notification service with multiple channels

• A send({ userId, type, data }) method delivers a notification through the channels that the user has enabled for that type (email, SMS, push, in-app)
• Each channel is an adapter implementing the same interface (send(recipient, message)), so a new provider or channel needs no change to the core
• Messages are produced from templates with placeholders (Hello {{name}}), with a template per type and channel and a clear error for missing variables
• User preferences control channels per notification type, plus quiet hours; critical types bypass preferences
• Failed deliveries are retried with exponential backoff up to a limit, then recorded as failed; a failure in one channel does not stop the others
• A per-user, per-channel rate limit (for example 5 per minute, token bucket or sliding window) drops or delays excess notifications, with an injectable clock for tests

Only after every requirement works.

• Fallback to another channel when the preferred one fails
• Batch or digest notifications
• Deduplicate with an idempotency key