Feat: Add User Authentication and Authorization System (Signup/Login)

## Is your feature request related to a problem? Please describe.

Currently, the Perspective application lacks user authentication and authorization mechanisms. This means:
- Users cannot create accounts or maintain personalized sessions
- There's no way to track individual usage or enforce rate limiting per user
- Saved analyses, chat history, and preferences cannot be persisted across sessions
- The application cannot scale to a production-ready system with proper access control
- No protection exists for API endpoints against abuse or unauthorized access

## Describe the solution you'd like

Implement a comprehensive authentication and authorization system with the following features:

**Backend (FastAPI):**
- User registration endpoint with email verification
- Secure login/logout functionality using JWT tokens
- Password hashing with bcrypt or argon2
- Token refresh mechanism for extended sessions
- Protected API routes with authentication middleware
- Rate limiting per authenticated user
- User profile management (view/update profile)

**Frontend (Next.js):**
- Clean signup/login forms with validation
- Secure token storage (httpOnly cookies or secure localStorage)
- Authentication state management (Context API or Zustand)
- Protected routes that redirect unauthenticated users
- User profile dashboard
- Session persistence and auto-refresh
- Logout functionality with token cleanup

**Database:**
- User table with fields: id, email, username, hashed_password, created_at, is_verified
- Integration with MongoDB database for user-specific data

## Potential Impact

**Who will benefit:**
- **End users** - Personalized experience, saved history, secure access
- **Project maintainers** - Better abuse prevention, usage analytics, scalable infrastructure
- **Contributors** - Foundation for advanced features (sharing, collaboration, premium tiers)
- **Community** - Production-ready system that can be deployed publicly

**Expected impact:**
- **Security**: Prevents API abuse and unauthorized access
- **Scalability**: Enables per-user rate limiting and resource management
- **User Experience**: Persistent sessions, saved preferences, analysis history
- **Monetization potential**: Foundation for freemium/premium features
- **Data privacy**: User-specific data isolation and GDPR compliance readiness
- **Analytics**: Track user engagement, popular features, and system usage

@ParagGhatage i have been working on the code for this issue. once this is approved by you i shall raise the pr.