feat: Complete STRIDE validation and Docker persistence improvements

[drneox]

✨ STRIDE Category Validation:

• Add stride_validator.py with normalize_stride_category function
• Implement STRIDE validation in API endpoints (manual + AI threats)
• Add validation in CRUD operations for threat updates
• Ensure data consistency across all threat creation/update paths

🐛 API Cleanup:

• Remove duplicate imports in api.py
• Fix indentation issues
• Remove obsolete commented code
• Organize imports by category (stdlib, third-party, local)

🗂️ File Management Improvements:

• Implement UUID-based filenames for uploaded diagrams
• Add file extension validation for security
• Return both base64 and saved filename from save_image()
• Prevent filename collisions and security issues

🐳 Docker Persistence:

• Add diagrams_data volume for persistent diagram storage
• Configure nginx to serve diagrams directly from volume
• Fix diagram loss on container restart/rebuild
• Remove obsolete docker-compose version attribute

🎨 Frontend Enhancements:

• Implement compact STRIDE dropdown (shows first letter only)
• Add ResidualRiskSelector pattern for better UX
• Maintain consistent STRIDE categories between frontend/backend

🛡️ Security & Consistency:

• All STRIDE categories normalized to standard format
• Case-insensitive validation with fallback to 'Spoofing'
• File type validation prevents malicious uploads
• Centralized validation logic in stride_validator module