perf(leaderboard): reduce builder/community "view all" latency by skipping validator-only counts (#378)

[ismailkonvah]

Summary

This PR improves performance for "View All Builders" and other non-validator leaderboard views by removing unnecessary validator-wallet counting work from the request path.

Problem

Issue #378 reports slow loading in builder-focused "view all" flows.
The leaderboard endpoint was always adding validator wallet count annotations, and serializer fallback logic could still query validator wallets even for non-validator leaderboard rows.

Changes

1) Optimize queryset annotations by leaderboard type

In LeaderboardViewSet.get_queryset:

• Moved type resolution earlier
• Applied validator wallet annotations only when type is validator-related:
  • validator
  • validator-waitlist
  • validator-waitlist-graduation
• Non-validator leaderboards (e.g. builder/community) now skip these expensive joins/counts.

2) Prevent serializer fallback queries on non-validator rows

In LeaderboardEntrySerializer.get_active_validators_count:

• Early-return None when entry type is not validator-related
• Keeps validator count behavior unchanged for validator leaderboards
• Avoids extra DB queries during serialization for builder/community lists

Why this helps

Builder/community "view all" requests no longer perform validator-specific counting logic, reducing query complexity and serialization overhead.

Scope

• No API contract changes for existing leaderboard fields
• Validator leaderboards keep active validator count behavior
• Non-validator leaderboards now avoid unnecessary count work

Issue

Closes #378

Validation

• Static check passed:
  • python -m py_compile backend/leaderboard/views.py backend/leaderboard/serializers.py