Implement automated invoice generation from milestone completion

[Smartdevs17]

Context

Invoices need automation from milestone completion.

Current Limitation

Invoices are manually created.

Expected Outcome

• Automated invoice generation
• Milestone-triggered invoicing
• Invoice delivery automation
• Invoice tracking

Acceptance Criteria

• Automated invoice generation
• Milestone-triggered invoicing
• Invoice delivery automation
• Invoice status tracking
• Invoice reminders
• Invoice analytics
• Invoice documentation