feat: Add complete monetization system

Backend:
- User authentication with JWT tokens (auth_service.py)
- Subscription plans: Free, Starter (), Pro (), Business (), Enterprise
- Stripe integration for payments (payment_service.py)
- Usage tracking and quotas
- Credit packages for pay-per-use
- Plan-based provider restrictions

Frontend:
- Landing page with hero, features, pricing preview (landing-sections.tsx)
- Pricing page with all plans and credit packages (/pricing)
- User dashboard with usage stats (/dashboard)
- Login/Register pages with validation (/auth/login, /auth/register)
- Ollama self-hosting setup guide (/ollama-setup)
- Updated sidebar with user section and plan badge

Monetization strategy:
- Freemium: 3 docs/day, Ollama only
- Starter: 50 docs/month, Google Translate
- Pro: 200 docs/month, all providers, API access
- Business: 1000 docs/month, team management
- Enterprise: Custom pricing, SLA

Self-hosted option:
- Free unlimited usage with own Ollama server
- Complete privacy (data never leaves machine)
- Step-by-step setup guide included

.env.example

@@ -73,6 +73,19 @@ ADMIN_PASSWORD=changeme123
 # Token secret for session management (auto-generated if not set)
 # ADMIN_TOKEN_SECRET=
 
+# ============== User Authentication ==============
+# JWT secret key (auto-generated if not set)
+# JWT_SECRET_KEY=
+
+# Frontend URL for redirects
+FRONTEND_URL=http://localhost:3000
+
+# ============== Stripe Payments ==============
+# Get your keys from https://dashboard.stripe.com/apikeys
+STRIPE_PUBLISHABLE_KEY=pk_test_...
+STRIPE_SECRET_KEY=sk_test_...
+STRIPE_WEBHOOK_SECRET=whsec_...
+
 # ============== Monitoring ==============
 # Log level: DEBUG, INFO, WARNING, ERROR
 LOG_LEVEL=INFO