[EXTRACT] QuantityUnitService - Rip quantity unit operations from monolith

[democratize-technology-code-reviewer]

🔪 Extraction Target

Extract all quantity unit operations from the 19,843-line index.mjs into a focused QuantityUnitService class. Final extraction.

📁 Methods to Extract

// All of these get ripped out and moved to QuantityUnitService
getQuantityUnits()
getQuantityUnit()
createQuantityUnit()
updateQuantityUnit()
deleteQuantityUnit()
getQuantityUnitConversions()
// ... and any other quantity-unit-related methods

🎯 Definition of Done

• Create services/QuantityUnitService.js extending BaseService
• Extract ALL quantity unit 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/QuantityUnitService.js
export class QuantityUnitService extends BaseService {
  async getQuantityUnits() {
    // Direct copy from index.mjs - no changes
  }
  
  async createQuantityUnit(unit) {
    // Direct copy from index.mjs - no changes
  }
  
  // ... rest of quantity unit methods
}

🚫 NOT in Scope

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

💀 Breaking Changes

Yes. Embrace change.

Related to

Parent issue: #11