fix: honor reminder job cancellation

[vycdev2]

Summary

• propagate Quartz cancellation through reminder loading, per-reminder processing, and persistence
• add a regression test proving pre-canceled reminder runs stop before database work

Verification

• dotnet build --no-restore — passed with 0 warnings and 0 errors
• dotnet test --no-restore --filter FullyQualifiedName~RemindersJobTests — passed, 12/12 tests
• dotnet test --no-restore --filter 'FullyQualifiedName!~McpCommandExecutionTests.Invocation_ValidatesLocaleAndTimeZoneContext' — passed, 503 passed and 1 skipped
• dotnet test --no-restore — 503 passed, 1 skipped, and 1 unrelated pre-existing failure because this runner forces globalization-invariant mode and cannot resolve en-US
• git diff --check — passed

Risk

• Low: cancellation now stops remaining reminder processing and prevents a canceled save; normal runs are unchanged.

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