feat: implement payroll calculation system with superannuation, pay slips & email notifications

[SinghAkashdeep16]

Summary

• Implement full payroll calculation engine with daily/weekly overtime rules (Australian Fair Work compliance)
• Add superannuation tracking at 11.5% (FY 2024-25 rate) per entry and as aggregate totals (totalSuperannuation, totalCostToEmployer)
• Add GET /:payrollId/payslip endpoint to generate and download PDF pay slips per guard
• Notify guards via email when their payroll is approved or processed (non-blocking, dev-mode terminal fallback)
• Extend Payroll model with totalRegularPay, totalOvertimePay, totalSuperannuation, totalCostToEmployer fields
• Extend period summary in getPayroll to include all new aggregates and superannuation rate

Test plan

• Generate payroll for a period and verify totalSuperannuation = 11.5% of grossPay
• Verify totalCostToEmployer = grossPay + totalSuperannuation
• Call GET /api/v1/payroll/:payrollId/payslip as admin — confirm PDF downloads correctly
• Call the same endpoint as a guard for their own record — confirm access allowed
• Call the same endpoint as a guard for another guard's record — confirm 403 returned
• Approve payroll and check terminal (dev mode) or inbox for approval email
• Process payroll and check terminal (dev mode) or inbox for processed email

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