[EXTRACT] StockService - Rip stock operations from monolith

[democratize-technology-code-reviewer]

🔪 Extraction Target

Extract all stock-related operations from the 19,843-line index.mjs into a focused StockService class. No backward compatibility. No mercy.

📁 Methods to Extract

// All of these get ripped out and moved to StockService
addToStock()
consumeFromStock()
inventoryProduct()
openProduct()
getStock()
getStockEntries()
getStockEntry()
getStockLocations()
getStockProductDetails()
transferProduct()
// ... and any other stock-related methods

🎯 Definition of Done

• Create services/StockService.js extending BaseService
• Extract ALL stock 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/StockService.js
export class StockService extends BaseService {
  async addToStock(productId, amount, bestBeforeDate, price, locationId, transactionType) {
    // Direct copy from index.mjs - no changes
  }
  
  async consumeFromStock(productId, amount, spoiled, locationId, recipeId, exact) {
    // Direct copy from index.mjs - no changes
  }
  
  // ... rest of stock methods
}

🚫 NOT in Scope

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

💀 Breaking Changes

Yes. Deal with it. Update your code.

Related to

Parent issue: #11