fix(community): optimize leaderboard/community endpoint to prevent 30s timeout (#383)

[ismailkonvah]

Summary

This PR fixes issue #383 where the Community page (Top Community Members) could fail with timeout of 30000ms exceeded.

Root Cause

The /api/v1/leaderboard/community/ endpoint serialized each user with full UserSerializer, which is expensive and can trigger slow/N+1 query behavior as data grows.

Changes

• Optimized community action in backend/leaderboard/views.py:
  • Replaced full UserSerializer usage with a lightweight values(...) projection for only required fields:
    • id, name, address, profile_image_url, builder_points, validator_points, total_points
  • Moved totals computation to DB aggregation (Sum) instead of Python loops
  • Added optional limit query param (default 200, clamped to 1..1000) to control payload size

Why this fixes it

The endpoint now performs less work per row and returns a smaller payload, significantly reducing response time and lowering timeout risk on the Community page.

Issue

Closes #383

Testing

• Static check: python -m py_compile backend/leaderboard/views.py passed
• Manual verification recommended:
  1. Open /community
  2. Confirm Top Community Members loads without timeout
  3. (Optional) Verify ?limit=50 returns bounded results