fix: avoid username check date overflow

[vycdev2]

Summary

• avoid DateTime.AddDays overflow when a stored username-check timestamp is near the supported limit
• reuse one UTC timestamp for the eligibility check and persisted update
• add a regression test and changelog entry

Verification

• dotnet test --no-restore --filter FullyQualifiedName~UsersServiceTests.TryUpdateUsername_DoesNotOverflowWhenNextCheckIsOutsideDateTimeRange — passed (1 test)
• dotnet test --no-restore — passed (555 passed, 1 skipped)
• dotnet build --no-restore — passed (0 warnings, 0 errors)
• python3 -m unittest tools.test_generate_commands_md — passed (3 tests)
• git diff --check upstream/develop...HEAD — passed

Risk

• Low: the ten-day eligibility rule is unchanged; only the comparison avoids constructing an out-of-range date.

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