feat: add subscription service — user auth, Stripe billing, API keys, dashboard
- NextAuth v5 with email+password credentials, JWT sessions
- Registration, login, email verification, password reset flows
- Stripe integration: Free (15/day), Starter ($5/1k/mo), Pro ($20/100k/mo)
- API key management (cb_ prefix) with hash-based validation
- Dashboard with generations history, settings, billing management
- Rate limiting: Redis daily counter (free), DB monthly (paid)
- Generate route auth: Bearer API key + session, anonymous allowed
- Worker userId propagation for generation history
- Pricing section on landing page, auth-aware navbar
- Middleware with route protection, CORS for codeboard.vectry.tech
- Docker env vars for auth, Stripe, email (smtp.migadu.com)
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