Implement User Authentication and Document Ownership

[Davz33]

Priority: Low

Add user management and document ownership features:

Current State:

• No user authentication
• All documents are public/global
• No access control

Expected Features:

• User registration and login
• Document ownership and privacy
• User-specific document collections
• Access control and permissions
• User activity tracking

Technical Approach:

• Add FastAPI security with JWT tokens
• Implement user database schema
• Add document ownership fields
• Create user management endpoints
• Implement access control middleware
• Add user dashboard and settings