fix(workers): stop database monitor during shutdown

[iamcracked-dev]

Closes #321
Closes #320
Closes #319
Closes #318

Summary

Completes the application shutdown path for the database connection-pool monitor so the process stops its periodic background work before database resources are released.

Changes

• Starts the database monitor with a cancellable context instead of an unbounded ticker loop.
• Tracks monitor completion and waits for it during shutdown.
• Stops the monitor after the existing HTTP and worker shutdown sequence, but before closing the SQL connection pool.
• Preserves the existing monitoring cadence and structured log fields while the service is running.

Scope

The repository already loads runtime configuration from environment variables and already coordinates its request workers through cancellation and a wait group. This focused change closes the remaining shutdown gap for the independent database monitor. The other assigned issues are listed above for assignment tracking; their broader API-validation and logging work remains separately scoped.

Verification

No dependencies were installed and no builds, tests, or CI checks were run, per request.

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