Implement subscription invoice customization with branding

[Smartdevs17]

Context

Invoices need customization with branding.

Current Limitation

Invoices are generic.

Expected Outcome

• Invoice branding customization
• Template management
• PDF generation
• Invoice analytics

Acceptance Criteria

• Invoice branding customization
• Template management
• PDF generation with branding
• Invoice analytics
• Invoice preview
• Invoice API
• Invoice documentation