refactor(worker): Terminate the SMS cluster worker infinite setTimeout loop and gate demo seeding

[SharifIbrahimDev]

Closes #918

Summary of Changes

• Gated demo SMS queue seeding behind environment flags (process.env.NODE_ENV === 'development' or process.env.SEED_DEMO_SMS === 'true') to prevent unconditional demo data injection on module import.
• Added stopSMSClusterWorker() lifecycle method with clearable timer and isRunning flag.
• Called .unref() on the polling timer in Node.js environment to prevent blocking process exit.
• Added unit tests in src/__tests__/sms-cluster-worker.test.ts.

Technical Requirements Checklist

• Clearable timer and shutdown control (stopSMSClusterWorker).
• Timer .unref() to avoid blocking process exit.
• Gate demo queue seeding behind explicit development environment check.

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