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

requirements.txt

@@ -7,6 +7,7 @@ python-pptx==0.6.23
 deep-translator==1.11.4
 python-dotenv==1.0.0
 pydantic==2.5.3
+pydantic[email]==2.5.3
 aiofiles==23.2.1
 Pillow==10.2.0
 matplotlib==3.8.2
@@ -18,3 +19,9 @@ openai>=1.0.0
 # SaaS robustness dependencies
 psutil==5.9.8
 python-magic-bin==0.4.14  # For Windows, use python-magic on Linux
+
+# Authentication & Payments
+PyJWT==2.8.0
+passlib[bcrypt]==1.7.4
+stripe==7.0.0
+