Add Discord notifications (single status + per-area summary) to unified CI

Description

Enhance the unified CI workflow to post one Discord message per run (pass/fail/cancelled), including a brief per-area result summary (backend/frontend). Uses a repository secret DISCORD_WEBHOOK_URL.

Motivation

• One clean message per run for better signal/noise
• Centralized status across areas

Prerequisites

Create a Discord webhook and add a GitHub Actions secret: Name: DISCORD_WEBHOOK_URL Value: (Discord webhook URL)

Scope

• Add a final notify job that:
  • Depends on all per-area jobs (and runs even if they fail)
  • Aggregates overall conclusion + per-area statuses
  • Posts one Discord message to the webhook
  • Update CONTRIBUTING.md with setup/behavior details for the webhook and notification format

Out of Scope

• Changing build/test commands
• Per-environment deploys

Acceptance Criteria

• A single Discord message is posted per workflow run with:
  • Overall status (pass/fail/cancelled)
  • Per-area results (backend/frontend) shown clearly
  • Link to the run/PR
• CONTRIBUTING.md documents the webhook secret and how notifications work

Docs Updates

• In CONTRIBUTING.md, add:
  • How to create & store the DISCORD_WEBHOOK_URL secret
  • What the message looks like and when it sends
  • Troubleshooting (e.g., missing secret, non-200 response)