fix: serialize reminder delivery runs

[vycdev2]

Summary

• prevent overlapping Quartz reminder runs from delivering the same due reminder more than once
• add regression coverage requiring reminder delivery jobs to disallow concurrent execution

Verification

• dotnet build — passed with 0 errors (2 existing NU1903 warnings)
• dotnet test --filter 'FullyQualifiedName~RemindersJobTests' --logger 'console;verbosity=minimal' — passed 11/11
• dotnet test --no-build --logger 'console;verbosity=minimal' — passed 429, skipped 1, failed 0
• git diff --check — passed

Risk

• Low: this only serializes executions of the same Quartz reminder job and does not change reminder selection, delivery, or retry behavior.

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