fix: prevent global average count overflow

[vycdev2]

Summary

• promote global average-message leaderboard count aggregates to 64-bit before summing across guilds
• add a regression covering display and viewer rank when a user total exceeds int.MaxValue

Verification

• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --filter FullyQualifiedName~GetGlobalAverageLengthLeaderboardAsync_SupportsMessageTotalsLargerThanIntMaxValue — passed: 1/1 (the regression failed with OverflowException before the fix)
• dotnet build — passed with 0 errors; 2 existing NU1903 warnings
• dotnet test --no-build — passed: 429, skipped: 1, failed: 0
• git diff --check upstream/develop...HEAD — passed

Risk

• Low: the change only widens aggregate message counts used by global average-length calculations and ranking; persisted column types and command behavior are unchanged.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.