input: Write a python function to design a database schema for a sports event management system.
  assert 'Users' in design_database_schema()
  assert 'Sports' in design_database_schema()
  assert 'Venues' in design_database_schema()
  assert 'Events' in design_database_schema()
output: def design_database_schema():
      schema = {
          'Users': {
              'user_id': 'INTEGER PRIMARY KEY',
              'name': 'TEXT',
              'email': 'TEXT UNIQUE',
              'password': 'TEXT',
              'role': 'TEXT'  # e.g., 'admin', 'participant', 'organizer'
          },
          'Sports': {
              'sport_id': 'INTEGER PRIMARY KEY',
              'name': 'TEXT UNIQUE',
              'rules': 'TEXT'
          },
          'Venues': {
              'venue_id': 'INTEGER PRIMARY KEY',
              'name': 'TEXT UNIQUE',
              'location': 'TEXT',
              'capacity': 'INTEGER'
          },
          'Events': {
              'event_id': 'INTEGER PRIMARY KEY',
              'name': 'TEXT',
              'sport_id': 'INTEGER',
              'venue_id': 'INTEGER',
              'date': 'TEXT',
              'time': 'TEXT',
              'organizer_id': 'INTEGER',
              'FOREIGN KEY (sport_id)': 'REFERENCES Sports(sport_id)',
              'FOREIGN KEY (venue_id)': 'REFERENCES Venues(venue_id)',
              'FOREIGN KEY (organizer_id)': 'REFERENCES Users(user_id)'
          }
      }
      return schema