[EXTRACT] ProductService - Rip product operations from monolith

[democratize-technology-code-reviewer]

🔪 Extraction Target

Extract all product operations from the 19,843-line index.mjs into a focused ProductService class. Direct migration.

📁 Methods to Extract

// All of these get ripped out and moved to ProductService
getProducts()
getProduct()
createProduct()
updateProduct()
deleteProduct()
getProductBarcodes()
addProductBarcode()
// ... and any other product-related methods

🎯 Definition of Done

• Create services/ProductService.js extending BaseService
• Extract ALL product methods from index.mjs
• File size < 500 lines (enforced by linter)
• All existing tests pass WITHOUT modification
• Add JSDoc comments for each method
• No deprecated methods or compatibility layers
• Update all imports/references in index.mjs
• Zero regression - identical functionality

⚡ Implementation

// services/ProductService.js
export class ProductService extends BaseService {
  async getProducts() {
    // Direct copy from index.mjs - no changes
  }
  
  async createProduct(product) {
    // Direct copy from index.mjs - no changes
  }
  
  async addProductBarcode(productId, barcode) {
    // Direct copy from index.mjs - no changes
  }
  
  // ... rest of product methods
}

🚫 NOT in Scope

• Backward compatibility
• Deprecation warnings
• Proxy methods in GrocyClient
• Changing method signatures
• Adding new features

💀 Breaking Changes

Yes. Modernize.

Related to

Parent issue: #11