fix: add graceful shutdown drain for /api/keys

[chiscookeke11]

Summary

This PR adds a graceful shutdown drain handler for /api/keys\ requests upon receiving SIGTERM or SIGINT signals.

Closes #943

Details of Changes

• Integrated \keysDrainTracker\ (\createInFlightDrainTracker('api-keys')) to monitor active /api/keys\ requests.
• Added subsystem shutdown tracking so in-flight requests finish before server exit.
• Configured incoming requests during server drain phase to receive \Connection: close\ headers.
• Verified zero regressions with unit tests covering shutdown drain behavior.

Verification

Ran
px jest src/lifecycle/shutdown.test.ts\ to confirm all 21 shutdown tests pass successfully:

• Active /api/keys\ requests finish before process exit.
• \Connection: close\ header set on new requests arriving during drain phase.

[greatest0fallt1me]

Green ✅ merging. Thanks for the contribution!