Add Docstrings to Public Functions

[Ndukiye]

Description:
Many functions lack docstrings. Add Google-style docstrings to:

• storage.py: get_date_range_usage(), cleanup_old_history()
• enforcer.py: enforce_limit(), notify_user()
• Monitor.py: normalize_bpftrace_line(), get_process_owner()

Example:

def get_date_range_usage(start_date, end_date):
    """
    Get aggregated bandwidth usage across a date range.
    
    Args:
        start_date (datetime.date): Start of range (inclusive)
        end_date (datetime.date): End of range (inclusive)
    
    Returns:
        dict: Process names mapped to {send, recv, total} in MB
    
    Example:
        >>> usage = get_date_range_usage(date(2024,1,1), date(2024,1,7))
        >>> usage['firefox']['total']
        1250.5
    """

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