[EXTRACT] FileService - Rip file operations from monolith

[democratize-technology-code-reviewer]

🔪 Extraction Target

Extract all file operations from the 19,843-line index.mjs into a focused FileService class. Direct transfer.

📁 Methods to Extract

// All of these get ripped out and moved to FileService
uploadFile()
downloadFile()
deleteFile()
getFiles()
// ... and any other file-related methods

🎯 Definition of Done

• Create services/FileService.js extending BaseService
• Extract ALL file 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/FileService.js
export class FileService extends BaseService {
  async uploadFile(entity, entityId, fileName, fileData) {
    // Direct copy from index.mjs - no changes
  }
  
  async downloadFile(group, fileName) {
    // Direct copy from index.mjs - no changes
  }
  
  // ... rest of file methods
}

🚫 NOT in Scope

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

💀 Breaking Changes

Yes. Move forward.

Related to

Parent issue: #11