Implement Atomic Payroll Finalization Workflow #1902

[revatikadam0607]

This PR introduces an atomic payroll finalization workflow that ensures
all database mutations belonging to payroll finalization succeed or roll
back together, eliminating partial finalization states.

Changes include:

1. New PayrollFinalizationService - coordinates all finalization operations
   within a MongoDB transaction
2. Enhanced PayrollRun model - tracks finalization state and optimistic locking
3. Updated PayrollController - leverages atomic service for approval flow
4. Comprehensive tests - cover transaction failures and concurrent finalization

Addresses Issue #1902:
✓ Prevents partial payroll finalization
✓ Safely handles concurrent finalization attempts
✓ Provides recovery handling for failed operations
✓ Includes optimistic locking for concurrent safety
✓ Schedules background jobs only after successful persistence

Closes#1902
@Dev1822

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