fix: stabilize activity leaderboard pagination

[vycdev2]

Summary

• add a stable user-ID tie-breaker to every paginated activity leaderboard query
• add a regression test covering equal-XP users split across page boundaries

Verification

• dotnet build — passed (0 errors; existing NU1903 advisory warning)
• dotnet test — passed (227 tests)
• git diff --check — passed

Risk

• Low: only ordering among users tied on the displayed metric changes; no schema or command changes.

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