refactor(invoices): extract payload processing to InvoiceProcessor service

[nikhilb2]

Summary

Moved invoice payload application and inventory management logic out of the route handler into a dedicated InvoiceProcessor service class in backend/src/services/invoice_processor.py.

The previous _apply_payload_to_invoice() function was ~275 lines of deeply nested logic with multiple responsibilities — ledger lookup, item validation, inventory checks, tax calculation, and ORM persistence — all living in the route file, making it untestable in isolation.

Type of change

• chore/refactor

How to test

cd backend
DATABASE_URL=postgresql://... python -m pytest tests/ -v

All 165 existing tests continue to pass, and 27 new unit tests cover the extracted service.

Checklist

• My code follows the project style and conventions
• I added/updated tests where appropriate
• I ran relevant checks locally
• I verified this does not break existing behavior

What changed

New file: backend/src/services/invoice_processor.py

• InvoiceProcessor class with db: Session injected in __init__
• apply_payload(invoice, payload, active_company, ...) — main entry point (replaces _apply_payload_to_invoice)
• validate_items(items, company_id, voucher_type, ...) — item validation extracted
• calculate_totals(validated_items, tax_inclusive) — tax/total calculation extracted
• apply_inventory_delta_for_update(invoice, payload, *, company_id) — net delta for edits
• reverse_inventory(invoice) — used by cancel route
• restore_inventory(invoice, *, company_id) — used by restore route
• Module-level helpers: inventory_effect_for_voucher_type(), change_inventory_quantity()

Modified: backend/src/api/routes/invoices.py

• Removed all moved logic (~275 lines)
• Route handlers now create InvoiceProcessor(db) and delegate

New test file: backend/tests/services/test_invoice_processor.py

• 27 focused unit tests covering every method and edge case

Updated tests (patch targets moved from invoices._generate_next_number → invoice_processor.generate_next_number):

• test_invoice_update_inventory.py
• test_invoice_reference_notes.py
• test_invoice_dues_allocations.py
• test_invoice_tax_split.py (now uses InvoiceProcessor.apply_payload)

Related issue

Closes #351