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>

config/initializers/assets.rb

@@ -5,3 +5,6 @@ Rails.application.config.assets.version = "1.0"
 
 # Add additional assets to the asset load path.
 # Rails.application.config.assets.paths << Emoji.images_path
+
+# Add build output directory to asset load path for Propshaft
+# Rails.application.config.assets.paths << Rails.root.join("app/assets/builds")