feat: add webhook event system for financial events (Bounty #77)

[zp6]

Webhook Event System

Closes #77

Implementation

• Type-safe event bus with TypeScript generics
• Local event handlers with unsubscribe support
• External webhook delivery with retry logic
• HMAC secret verification via headers
• Configurable retry count and delay
• Comprehensive test suite

Events Supported

• transaction.created/updated/deleted
• budget.threshold_reached/exceeded
• goal.completed
• report.generated
• account.connected/disconnected
• alert.triggered

Usage

const system = new WebhookEventSystem({ retryCount: 3 })

// Local handler
system.on('transaction.created', (event) => {
  console.log(event.data)
})

// External webhook
system.registerWebhook('slack', {
  url: 'https://hooks.slack.com/...',
  secret: 'my-secret',
  events: ['budget.exceeded'],
  active: true,
})

// Emit
await system.emit('transaction.created', { id: '123', amount: 100 })

Wallet: zp6