feat(feed): add Atom RSS feed for public profile activity

[mallya-m]

What does this PR do?

Adds a valid Atom XML feed at /u/<username>/feed.xml for public DevTrack profiles, allowing developers to syndicate their GitHub activity to RSS readers and portfolio sites.

Related issue

Closes #944

Changes made

• New route: src/app/u/[username]/feed.xml/route.ts
• Returns valid Atom XML with Content-Type application/atom+xml
• Reuses existing formatActivity from activity-formatter.ts — no new DB queries
• Returns 404 if user profile is not public (respects visibility settings)
• Feed contains last 20 activity items (pushes, PRs, releases, issues)
• Each entry has: title, link, published date, summary
• No new dependencies — template literal XML only
• 5-minute Cache-Control for RSS reader polling

How to test

1. Enable public profile in dashboard settings
2. Visit /u/<your-github-login>/feed.xml
3. Valid Atom XML returned with recent activity items
4. Disable public profile — returns 404

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[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

[mallya-m]

@Priyanshu-byte-coder could you please add gssoc:approved, level:intermediate, and type:feature labels? Thank you!