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Build a lightweight behavioral email automation engine in Node.js (TypeScript) using Express, Prisma, PostgreSQL, and BullMQ.

1. DATA MODEL:
- User: id, email, properties (JSONB), createdAt
- Event: id, userId, eventName, payload (JSONB), timestamp
- Sequence: id, name, triggers (array of event names), isActive
- SequenceStep: id, sequenceId, stepOrder, delayHours, templateSlug
- UserSequenceState: id, userId, sequenceId, currentStepId, nextExecuteAt, status ('active', 'completed', 'cancelled')

2. CORE FUNCTIONALITY:
- Endpoint POST /api/v1/events: Ingests user events. Saves the event to the database and evaluates active Sequences matched to the eventName.
- When a sequence matches: If the user is not already in the sequence, create a UserSequenceState record, find the first SequenceStep, and schedule a BullMQ job delayed by `delayHours`.
- Worker Engine: Processes delayed email jobs. It checks if the user's status is still 'active' and if they haven't unsubscribed. It renders the email template (using Handlebars with user properties and event payload) and sends it via Postmark API.
- Upon successfully sending, automatically advance the UserSequenceState to the next step, scheduling another delayed job if a subsequent step exists.
- Endpoint POST /api/v1/unsubscribe: Accepts a signed JWT to set an unsubscribe flag on the User record, cancelling pending BullMQ jobs.

3. FAILURE MODES & RESILIENCY:
- Handle rate limits and transient errors from the email provider by utilizing BullMQ retry mechanisms with exponential backoff (up to 3 attempts).
- Handle duplicate webhooks by implementing an idempotency key cache in Redis for incoming event POST requests.
- If a user triggers a sequence-cancellation event (e.g., 'order.placed' cancels an 'abandoned.cart' sequence), immediately mark the UserSequenceState as 'cancelled'.

4. OUT OF SCOPE:
- Visual drag-and-drop workflow nodes (define sequence steps in JSON or plain database tables).
- Visual HTML drag-and-drop template designer (use static raw HTML files or handlebars templates stored on disk).

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