feat: Outgoing Webhook support for new proposals

[mhoshdev]

Why

To allow maintainers to receive real-time notifications (e.g., in Slack or Discord) whenever a new feature is proposed, enabling faster moderation and response times.

Description

Implement an outgoing Webhook system that triggers a POST request to a configured URL whenever a new proposal is submitted. This bridges the gap between the feedback board and the team's existing communication tools.

Proposed Changes

• Support a WEBHOOK_URL environment variable.
• Implement an asynchronous HTTP POST request within the proposal creation API endpoint.
• Define a standard JSON payload (e.g., title, description, and link to the proposal).
• (Optional) Add a simple retry mechanism or error logging for failed deliveries.