Connect real Stripe test-mode + pricing page

[rz1989s]

Context

Billing infrastructure uses mock Stripe provider. Real Stripe test-mode would validate business model.

What

1. Replace mock Stripe with real Stripe test-mode SDK
2. Static pricing page at /pricing
3. "Sign up for free tier" flow issuing real API keys
4. Stripe Checkout for Pro/Enterprise upgrades

Judge Quotes

• Matty (79.5): "A 'Sign up for free tier' flow would transform this from a demo into a product."
• Kyle (77.7): "The billing system is infrastructure theater."

Impact

• Score impact: +2-3 pts across 2 judges (Matty, Kyle)
• Estimated time: 4-6 hours

[rz1989s]

Mock Stripe provider is sufficient for now — no need to connect real Stripe test-mode. The billing/metering architecture (S6-03) exists as a demo. Will revisit when actual monetization is needed.