fix: propagate xkcd job cancellation

[vycdev2]

Summary

• Propagate Quartz cancellation through the xkcd feed job database, feed-fetch, delivery, and retry operations.
• Re-throw caller cancellation instead of logging shutdown as a feed failure.
• Add a regression test covering cancellation during feed retrieval.

Verification

• dotnet restore Morpheus.sln — passed; emitted the existing NU1903 SQLitePCLRaw vulnerability warning.
• dotnet build Morpheus.sln --no-restore — passed with 1 existing NU1903 warning and 0 errors.
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --filter FullyQualifiedName~XkcdJobTests --no-restore --no-build -v q — passed: 10 tests.
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-restore --no-build -v q — 298 passed, 2 pre-existing MiscModuleTests failures for CC BIRTHDAY and CHRISTMAS under the invariant-globalization environment.
• git diff --check — passed.

Risk

• Low: cancellation tokens are threaded through one Quartz job without changing successful delivery or retry behavior.

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