fix: honor temporary ban job cancellation

[vycdev2]

Summary

• propagate Quartz cancellation through the temporary-ban database query, Discord unban request, processing loop, and final save
• rethrow caller cancellation instead of treating it as a retryable per-ban failure
• add a regression test covering cancellation before due bans are loaded

Verification

• dotnet build — passed (0 errors; 2 existing SQLitePCLRaw vulnerability warnings)
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --filter 'FullyQualifiedName~TemporaryBansJobTests' --logger 'console;verbosity=minimal' — passed (4/4)
• dotnet test --no-build --logger 'console;verbosity=minimal' — passed (429 passed, 1 skipped)
• git diff --check — passed

Risk

• Low: the change only affects shutdown/cancellation behavior for the scheduled temporary-ban job; normal unban and retry paths retain their existing behavior.

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