fix: align batchService.js stage mapping with constants/stages.js

[navin-oss]

Fixed the stage inconsistency between constants/stages.js and batchService.js.

Issue: The stage mappings were completely different:

• constants/stages.js: ['farmer', 'mandi', 'transport', 'retailer'] with mapping { farmer: 0, mandi: 1, transport: 2, retailer: 3 }
• batchService.js: { 'farmer': 0, 'processor': 1, 'distributor': 2, 'retailer': 3, 'consumer': 4 } (hardcoded inline)

Fix applied:

1. Imported STAGE_TO_NUMBER from constants/stages.js
2. Replaced the hardcoded inline stageMap with the imported STAGE_TO_NUMBER

This now correctly matches the Solidity contract's Stage enum (Farmer=0, Mandi=1, Transport=2, Retailer=3), preventing blockchain sync failures.