[FEATURE] Add Webhook Support

[JoshuaAFerguson]

Objective

Allow users to register webhooks for event notifications.

Events

• session.started
• session.stopped
• session.error
• session.idle
• user.created
• template.created

API Endpoints

POST /webhooks - Register webhook
GET /webhooks - List webhooks
DELETE /webhooks/{id} - Delete webhook
GET /webhooks/{id}/deliveries - Delivery history

Webhook Format

POST https://customer.com/webhook
X-StreamSpace-Event: session.started
X-StreamSpace-Signature: sha256=...
{
  "event": "session.started",
  "session_id": "123",
  "timestamp": "2025-11-23T10:00:00Z"
}

Acceptance Criteria

• Webhook CRUD endpoints
• Event delivery with retries
• HMAC signature verification
• Delivery history tracking
• Documentation

Files

• api/internal/webhooks/ (NEW)
• api/internal/events/ (NEW)