feat: add SSE stream endpoint with GitHub webhook and polling fallback

[devendra-w]

Summary

Adds a Server-Sent Events (SSE) stream endpoint with GitHub webhook integration and polling fallback for real-time dashboard updates.

Closes #237

Type of Change

• New feature

Changes Made

• Added src/app/api/stream/route.ts - SSE endpoint with auth check, session ownership verification, and cleanup on disconnect
• Added src/app/api/webhooks/github/route.ts - GitHub webhook handler that pushes events to connected SSE clients
• Added src/lib/sse.ts - in-memory SSE connections map
• Added src/components/SSEListener.tsx - client component that connects to the SSE stream with polling fallback
• Added test/github-webhook.test.ts - unit tests for webhook handler
• Updated src/app/dashboard/page.tsx - added SSEListener with session.githubId

How to Test

1. Sign in and navigate to /dashboard
2. Open browser DevTools → Network tab
3. Look for a request to /api/stream with type eventsource
4. Verify it stays connected

Checklist

• Linked issue in summary
• Self-reviewed the diff
• Added tests

[vercel]

@devendra-w is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[devendra-w]

Hi @Priyanshu-byte-coder! This is a clean rebase of #299 on the latest main. All CI checks are passing. Ready for review!

[devendra-w]

@Priyanshu-byte-coder could you please re-add the gssoc:approved and difficulty labels to this PR? This is a clean rebase of the previously approved PR.