Add Discount Management and Application to Billing System

[sweep-ai]

This pull request was created by Sweep to resolve the following request by @curtisdelicata:

Description: Implement a system for applying discounts and promo codes to invoices.

Tasks:
Create Discount model and migration.
Develop a UI for creating and managing discounts.
Implement functionality to apply discounts to invoices.
Acceptance Criteria:
Users can create and apply discounts or promo codes to invoices.
Discounts are reflected in the invoice total.

Continue chatting at https://sweep-chat-demo.vercel.app/c/a5463bfe-7e29-4a25-be54-74cc28d588ad.

Purpose

Implement a comprehensive discount management system for invoices, allowing creation, validation, and application of discounts across the billing workflow.

Description

This pull request introduces a complete discount management feature to the billing system. The implementation includes:

• A new Discount model with robust validation and usage tracking
• Database migration to support discount functionality
• Updated Invoice model to incorporate discount-related fields
• A BillingService method to apply discounts to invoices
• A DiscountController for managing discount CRUD operations
• Validation for discount codes, including time-based and usage-based restrictions

The discount system supports two types of discounts:

1. Percentage-based discounts
2. Fixed-amount discounts

Key features include:

• Unique discount codes
• Start and end date validity
• Maximum usage limits
• Currency support
• Automatic discount application and tracking

Summary

• Added new Discount model with comprehensive attributes
• Created database migration for discounts table
• Updated Invoice model with discount-related methods
• Implemented BillingService::applyDiscount() method
• Added DiscountController for discount management
• Introduced discount validation and application logic
• Supported percentage and fixed-amount discounts
• Added validation for discount code usage