Fix/issue 2087 report generation memory leak

[Vachhani-Tapan]

Fix: Memory Leak in Report Generation Service

Resolves#2087

Description

This PR fixes a critical memory leak that caused Node.js heap exhaustion (OOM/HTTP 502) when generating the Annual Consolidated Payroll Summary for large enterprise tenants.

Previously, the process used Model.find().lean() to buffer all records into memory synchronously.

Code Changes

• Created backend/src/services/reporting.service.js containing generateAnnualConsolidatedPayrollPDF().
• Replaced .lean() with MongoDB cursors (.cursor()).
• Piped pdfkit's PDFDocument directly to the provided outputStream (e.g., an Express res object).
• Processes documents sequentially via for await, rendering them immediately to the PDF stream and discarding them from RAM.

Verification

• Tested streaming a large mock dataset to verify heap usage remains flat.
• Ensured stream is properly closed on success and on error.

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