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

.gitignore

@@ -40,9 +40,11 @@ outputs/
 temp/
 translated_files/
 translated_test.*
+data/
 
 # Logs
 *.log
+logs/
 
 # UV / UV lock
 .venv/