fix: preserve Twitch state on API failure

[vycdev2]

Summary

• Distinguish Twitch API polling failures from a successful empty live-stream result.
• Preserve existing subscription live state when Twitch status cannot be fetched, avoiding false offline transitions.
• Pass the Quartz cancellation token through the live-status request.

Verification

• dotnet restore Morpheus.sln — passed with existing NU1903 SQLite vulnerability warning.
• dotnet build Morpheus.sln --no-restore -v minimal — passed with the same NU1903 warning.
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-build --no-restore --filter FullyQualifiedName~TwitchServiceTests -v minimal — passed (7 tests).
• Full dotnet test — 298 passed, 2 failed; the same two invariant-globalization failures reproduce on unchanged upstream/develop (baseline: 297 passed, 2 failed).
• dotnet format Morpheus.sln --no-restore --verify-no-changes — skipped as a publish gate: the unchanged repository has broad pre-existing CRLF/import/final-newline violations.

Risk

• Low: failed Twitch polling now leaves state untouched for the next scheduled poll; successful responses retain existing behavior.

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