feat: Complete backend implementation for Ledger module

[roncodes]

Overview

This PR implements the complete backend functionality for the Fleetbase Ledger accounting and invoicing module.

What's Included

Database Schema

• ledger_accounts: Chart of accounts with support for asset, liability, equity, revenue, and expense account types
• ledger_journals: Double-entry journal entries linking transactions to debit/credit accounts
• ledger_invoices: Invoice management with customer relationships and order integration
• ledger_invoice_items: Line items for invoices with tax calculation
• ledger_wallets: Wallet system for managing entity balances (drivers, vendors, etc.)

Models

• Full Eloquent models with relationships and traits
• Proper use of HasUuid, HasPublicId, HasApiModelBehavior
• Polymorphic relationships using subject_uuid and subject_type (new naming convention)
• All monetary values stored as integers (cents) for precision

Services

• LedgerService: Core double-entry bookkeeping logic
• WalletService: Wallet deposit, withdrawal, and transfer operations
• InvoiceService: Invoice creation from orders and payment recording

Controllers & Resources

• AccountController: CRUD operations for chart of accounts
• InvoiceController: Full invoice lifecycle management
• WalletController: Wallet management and transactions
• TransactionController: System-wide transaction viewer
• API resources for all models

Events & Observers

• InvoiceCreated and InvoicePaid events
• InvoiceObserver for lifecycle hooks

Key Features

✅ Double-entry bookkeeping system
✅ System-wide transaction viewing
✅ Wallet functionality for driver/entity payments
✅ Invoice generation from FleetOps orders
✅ Integration with core Transaction model
✅ Proper use of new subject naming convention

Testing

• All routes configured and ready for testing
• Service provider properly registers observers and services
• Follows Fleetbase architecture patterns

Next Steps

• Frontend implementation (Ember.js components and routes)
• PDF generation for invoices
• Email notifications
• Payment gateway integration