feat: implement custom leaderboard timeframes

[KrutagyaKaneria]

📌 Summary

This PR introduces support for custom leaderboard timeframes, allowing users to view rankings based on recent activity instead of only cumulative contributions.

The leaderboard now supports:

• 📅 Weekly (last 7 days)
• 📆 Monthly (last 30 days)
• 🏆 All-Time

This improves leaderboard competitiveness, highlights recently active contributors, and increases community engagement.

Closes #3045

---

🚀 Changes Made

Backend

• Added timeframe query parameter support to the leaderboard API.
• Added validation for supported values:
  • weekly
  • monthly
  • all_time
• Implemented timeframe-based aggregation logic.
• Extended leaderboard caching to maintain separate cache entries for each timeframe.
• Updated leaderboard rebuild logic to refresh all timeframe caches independently.

Frontend

• Added a segmented timeframe selector:
  • Weekly
  • Monthly
  • All-Time
• Connected UI to the updated leaderboard API.
• Added loading states while switching between timeframes.
• Preserved the existing leaderboard layout and responsiveness.

Performance

• Reused existing cache infrastructure.
• Avoided duplicate aggregation logic.
• Kept database queries optimized.
• Maintained backward compatibility with the existing leaderboard implementation.

---

✅ Testing

Verified the following:

• Weekly leaderboard
• Monthly leaderboard
• All-Time leaderboard
• Invalid timeframe returns 400 Bad Request
• Cache entries remain isolated per timeframe
• Leaderboard updates correctly when switching tabs
• Existing functionality remains unaffected

---

🛠️ Quality Checks

• pnpm run lint
• pnpm run type-check
• pnpm run test
• pnpm run build

---

Close #3045

[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

[KrutagyaKaneria]

@Priyanshu-byte-coder can you review my PR