fix: preserve activity level precision

[vycdev2]

Summary

• preserve fractional precision in the activity-level formula instead of truncating XP before applying the logarithm
• add regression coverage for the level 2 boundary and maximum long XP
• backfill persisted user levels with a reversible EF Core data migration

Verification

• dotnet build — passed with 0 errors (existing NU1903 dependency warning remains)
• dotnet test --no-build — passed, 229/229 tests
• dotnet-ef migrations script 20260731075933_AddReminderDeliveryBackoff 20260801124127_RecalculateActivityLevels --no-build — passed; generated the forward migration script
• dotnet-ef migrations script 20260801124127_RecalculateActivityLevels 20260731075933_AddReminderDeliveryBackoff --no-build — passed; generated the rollback script
• git diff --check upstream/develop...HEAD — passed

Risk

• Medium: the migration updates persisted level values whose old integer-division result differs, but does not change the schema and includes a rollback calculation.

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