[EXTRACT] UserService - Rip user operations from monolith

[democratize-technology-code-reviewer]

🔪 Extraction Target

Extract all user-related operations from the 19,843-line index.mjs into a focused UserService class. No backward compatibility. Direct extraction.

📁 Methods to Extract

// All of these get ripped out and moved to UserService
getUsers()
getUser()
getUserFields()
createUser()
updateUser()
deleteUser()
getSelfUser()
// ... and any other user-related methods

🎯 Definition of Done

• Create services/UserService.js extending BaseService
• Extract ALL user 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/UserService.js
export class UserService extends BaseService {
  async getUsers() {
    // Direct copy from index.mjs - no changes
  }
  
  async getUser(userId) {
    // Direct copy from index.mjs - no changes
  }
  
  // ... rest of user methods
}

🚫 NOT in Scope

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

💀 Breaking Changes

Yes. Update your imports.

Related to

Parent issue: #11