feat(notification): implement subscription webhook system with event filtering (#926)

[Steddy0]

Summary

Closes #926. Implements the subscription webhook delivery system with event filtering and wildcard subscription support across backend notification services and the developer portal.

Changes Made

• Webhook Delivery Engine (backend/services/notification/webhook.ts): Complete webhook delivery pipeline with HMAC-SHA256 signature verification, exponential backoff retries, dead-letter queue (DLQ) support, 24-hour idempotency de-duplication, and auto-disabling on 410 Gone.
• Event Filtering & Wildcards (backend/services/notification/webhooks.ts, backend/services/webhook/): Wildcard matching support (*, subscription.*, payment.*, etc.) enabling merchants to subscribe to broad categories or specific lifecycle events.
• REST Management API (backend/services/notification/webhookManagementApi.ts): Unified CRUD endpoints for endpoint registration, updating, pausing/resuming, secret rotation with overlapping grace periods, testing, and delivery replay.
• Developer Documentation & OpenAPI Specs (developer-portal/): Complete integration guide with HMAC verification examples, payload schema reference, testing tools, and OpenAPI 3.1.0 specifications.

Pull Request Checklist

Quality Gates (All must pass before merge)

• Lint: Code passes ESLint checks cleanly
• Type Check: TypeScript compilation verified
• Tests: All unit and integration test suites pass (18 webhook tests, 35 developer portal tests)
• Build: Project builds successfully

Additional Requirements

• New code has appropriate TypeScript types
• No hardcoded secrets or credentials
• New features have corresponding tests
• Documentation updated

Closes #926

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This PR will not be mergeable until all quality gates pass.