PHASE3-Bill1: Stripe integration + 3 pricing tiers

[psypherion]

Setup Stripe payment processing:

• FREE tier: 5 gen/month, basic tone engine
• PRO tier ($9/month): 100 gen/month, multi-variant, skeptic toggle
• ENTERPRISE ($99/month): Unlimited, white-label
• Webhook endpoints (subscription.created, updated, deleted)
• Checkout session creation
• Subscription state management