ENHANCEMENT: Add comprehensive server-side input validation and sanitization

[Senthil455]

Description

Server-side input validation is minimal throughout the application. Most routes only check if parameters exist (non-null checks), but do not validate format, length, range, or sanitize input. This can lead to data integrity issues, security vulnerabilities, and poor user experience.

Current Validation (minimal)

# app.py - Registration route (line ~45)
name = request.form['name']
email = request.form['email']
password = request.form['password']
# No validation on name length, email format, password strength
# No sanitization

# app.py - create_quiz route (line ~810)
title = request.form['title']
# No validation on title length, character restrictions

Missing Validations

1. Registration: Email format validation, password strength requirements (min length, complexity), name length limits
2. Quiz creation: Title/description length limits, date range validation (availableto > availablefrom), difficulty validation
3. Questions: Question text length, option validation (no duplicate options), correct answer must match an option
4. Class creation: Class name format validation
5. General: Server-side re-validation of client-side validated fields (client-side can be bypassed)

Recommended Enhancement

1. Create a validation utility module with reusable validators:

   import re
   
   def validate_email(email):
       pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
       return re.match(pattern, email) is not None
   
   def validate_password_strength(password):
       return len(password) >= 8
   
   def sanitize_input(text, max_length=500):
       # Strip HTML tags, limit length, trim whitespace
       cleaned = re.sub(r'<[^>]*>', '', text)
       return cleaned.strip()[:max_length]

2. Apply validations consistently across all routes, returning appropriate flash messages for invalid input

3. Validate dates and numbers with proper type checking and range validation