docs: Add comprehensive invoice system implementation guide
- Document various invoice generation approaches (PDF, HTML-to-PDF, Stripe) - Compare Stripe Payment Intents vs Invoicing vs Checkout Sessions - Provide complete code implementation with models, controllers, services - Include phase-by-phase implementation strategy for current use case - Add testing, security, and deployment guidelines - Recommend hybrid approach: keep current checkout + post-payment invoices 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <noreply@anthropic.com>
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