[codex] add server-owned job creation timestamps

[TheGreatAion]

Summary

• add a server-owned ISO createdAt timestamp when jobs are created
• keep caller-supplied timestamps from overriding the generated value
• add focused service regression coverage for returned and stored timestamps

Why

The Prisma Job model defines createdAt, but the in-memory createJob service omitted it. This left newly created records without creation metadata and allowed a caller-supplied createdAt value to flow through the payload spread.

Demo

Watch the short job timestamp demo

Verification

• node --test src/tests/health.test.js src/tests/job-created-at.test.js
• git diff --check
• Demo MP4 decoded successfully with ffmpeg -i job-created-at-demo.mp4 -f null -

The repository-wide npm test command still fails before discovery because node --test src/tests treats the test directory as a module path. That inherited harness issue is already tracked separately in #2014.

Closes #3313

/claim #743