feat: implement contractor 1099-NEC/MISC year-end reporting and TIN validation engine (closes #1871)

[Aakif-Kohari]

Description

This PR implements a Contractor 1099-NEC/MISC Year-End Reporting & TIN Validation Engine. It introduces schemas to track annual non-employee compensation, TIN match statuses, and FIRE format drafts. The engine aggregates YTD contractor payments against the $600 threshold and categorizes amounts into NEC vs MISC boxes. A "Backup Withholding Guardrail" automatically applies the 24% backup withholding rate to future payments for contractors with unresolved TIN mismatches (B-Notice status). The frontend allows Finance/AP Admins to review payment accumulations, manage TIN validation statuses, and export IRS FIRE-format magnetic media files.

Fixes#1871

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Changes Made

• backend/src/models/contractor1099.model.js (New): Schemas for ContractorPaymentLedger, TINValidationRecord, and Form1099Draft.
• backend/src/utils/form1099Engine.utils.js (New): Engine for threshold evaluation, backup withholding calculation, and FIRE format generation.
• backend/src/controllers/contractor1099.controller.js (New): Endpoints for payment recording, TIN validation, and FIRE file generation.
• backend/src/routes/contractor1099.routes.js (New): Route definitions.
• frontend/src/pages/Form1099Dashboard.jsx (New): UI to monitor YTD accumulations, TIN statuses, and export FIRE files.

How Has This Been Tested?

• Tested locally on frontend
• Tested locally on backend

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

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