Fix IP filtering logic, remove dead code, and consolidate imports

[Copilot]

Addresses code review feedback from PR #170 on IP whitelist implementation. The original IP filtering used string comparison for CIDR ranges, which fails for IP addresses (e.g., "192.168.1.10" < "192.168.1.9" lexicographically).

Core Fixes

• IP filtering: Replace string gte/lte with proper ipaddress module checks. Use values_list('id', 'ip_address') to reduce query overhead. Prefix filter candidates by network size (/24 → 3 octets, /16 → 2 octets, /8 → 1 octet with trailing dot to prevent false matches).

• Unreachable code: Remove impossible else branch in update_user_problem_stats that checked total_submissions > 0 after incrementing it.

• Dead code removal: Delete 107 lines of commented-out update_user_assignment_stats function and related callback comments.

• Import consolidation: Move ipaddress and logging to module level. Create single logger instance, removing 12 redundant local imports.

• Error handling: Raise ValidationError on invalid IP prefix instead of silent pass-through.

Example

# Before: String comparison (broken)
queryset = queryset.filter(
    ip_address__gte=start_ip,
    ip_address__lte=end_ip
)

# After: Proper IP address checking
network = ipaddress.ip_network(ip_prefix, strict=False)
candidate_data = queryset.filter(
    ip_address__startswith=network_prefix
).values_list('id', 'ip_address')

valid_ids = [
    sub_id for sub_id, ip_addr in candidate_data
    if ipaddress.ip_address(ip_addr) in network
]
queryset = queryset.filter(id__in=valid_ids)

Testing

Added 14 test cases covering simple prefix matching, CIDR notation (/8, /16, /24), invalid input handling, and user stats updates across score transitions.

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