fix: honor Twitch job cancellation

[vycdev2]

Summary

• propagate Quartz cancellation through Twitch subscription loading, polling, announcement delivery, and persistence
• stop before mutating subscription state when cancellation has already been requested
• add regression coverage for pre-load and pre-announcement cancellation

Verification

• dotnet build — passed with 0 errors (2 existing NU1903 warnings)
• dotnet test --filter 'FullyQualifiedName~TwitchLiveJobTests' --logger 'console;verbosity=minimal' — passed 4/4
• dotnet test --no-build --logger 'console;verbosity=minimal' — passed 430, skipped 1
• git diff --check upstream/develop...HEAD — passed

Risk

• Low: changes are limited to cancellation propagation in the Twitch polling job and preserve normal execution behavior.

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