Webhook outbound: notify external systems on stream events

[greatest0fallt1me]

Description

Allow users to register outbound webhooks (signed) for stream lifecycle events delivered from backend.

Requirements and context

• Retry policy with exponential backoff.
• Signing secret per subscriber.

Suggested execution

• Fork the repository and create a branch (example below).
• Implement changes in StreamPay-Backend (TypeScript / Express per package.json).
• Keep the service secure, tested, and documented; prefer small, reviewable PRs.

git checkout -b feature/outbound-webhooks

Implementation targets

• Tables: webhook_subscriptions, webhook_deliveries.
• Delivery worker.

Tests and validation

• Tests: HMAC signature verification on sample payload.

• Run npm test and npm run lint in StreamPay-Backend.

• Cover edge cases; include test output and brief security notes in the PR description.

Guidelines

• Target minimum 95% test coverage on new or changed backend code (or justify gaps).
• Clear documentation (OpenAPI / README / inline docs as specified).
• Timeframe: 96 hours from assignment.

Example commit message

feat(backend): outbound signed webhooks for stream events

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