fix: propagate feed job cancellation

[vycdev2]

Summary

• propagate Quartz cancellation through RSS and YouTube feed jobs, including database queries, feed/avatar requests, webhook delivery, and per-subscriber dispatch loops
• preserve cancellation instead of treating shutdown as a transient network failure in YouTube avatar lookup
• add direct cancellation regression tests for both feed jobs and avatar lookup

Verification

• dotnet restore Morpheus.sln — passed; existing SQLitePCLRaw.lib.e_sqlite3 NU1903 warning
• dotnet build Morpheus.sln --no-restore — passed with 0 errors
• dotnet test Morpheus.Tests/Morpheus.Tests.csproj --no-restore --filter "FullyQualifiedName~RssFeedJobTests|FullyQualifiedName~YoutubeRssJobTests|FullyQualifiedName~YoutubeUtilsTests" — passed, 26/26
• dotnet test Morpheus.sln --no-restore — 300 passed, 2 failed because this runner uses globalization-invariant mode and cannot load tr-TR; the same 2 failures reproduced on upstream/develop (baseline: 297 passed, 2 failed)
• dotnet format Morpheus.sln --no-restore --verify-no-changes — failed on existing repository-wide CRLF/whitespace diagnostics; no formatting changes were applied
• git diff --check — passed

Risk

• Low: cancellation is threaded through existing async boundaries without changing normal delivery or persistence behavior.

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