[Backend] Add Discord and Slack Webhook Alert Dispatcher

[Edge-Explorer]

Description

Currently, price threshold alert triggers dispatch email messages to the user via Gmail SMTP. We want to extend our notification layer to support modern webhooks so users can receive real-time ticker alert dispatches directly to their Discord or Slack channels.

Requirements

1. Create Webhook Client:
   • Add a webhook dispatch utility service backend/app/services/webhooks.py.
   • Use httpx to send a POST payload containing alert details (ticker name, crossed price, and datetime).