[EXTRACT] BatteryService - Rip battery operations from monolith

[democratize-technology-code-reviewer]

🔪 Extraction Target

Extract all battery-related operations from the 19,843-line index.mjs into a focused BatteryService class. Pure extraction.

📁 Methods to Extract

// All of these get ripped out and moved to BatteryService
getBatteries()
getBattery()
createBattery()
updateBattery()
deleteBattery()
trackBatteryCharge()
getBatteryDetails()
// ... and any other battery-related methods

🎯 Definition of Done

• Create services/BatteryService.js extending BaseService
• Extract ALL battery 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/BatteryService.js
export class BatteryService extends BaseService {
  async getBatteries() {
    // Direct copy from index.mjs - no changes
  }
  
  async trackBatteryCharge(batteryId, chargedDate) {
    // Direct copy from index.mjs - no changes
  }
  
  // ... rest of battery methods
}

🚫 NOT in Scope

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

💀 Breaking Changes

Yes. Update your code.

Related to

Parent issue: #11