Write API documentation for webhook endpoints and event schemas

[geminimir]

Summary

API documentation covering the webhook management REST API and all six event type schemas.

Changes

• OpenAPI/Swagger spec for webhook endpoint management API
• Event schema documentation for all 6 event types
• Example payloads for each event type
• Signature verification guide with code samples (Node.js, Python, Go)

Tests

• Example payloads validate against their schemas

Acceptance Criteria

Complete API and event documentation for integrators.