Add CIDR range support and scan scheduling

[stabrea]

Summary

Add support for scanning CIDR ranges (e.g., 192.168.1.0/24) and scheduled/recurring scans for continuous network monitoring.

Motivation

• Network security assessments typically target entire subnets, not individual hosts
• CIDR notation is the standard way to specify network ranges
• Scheduled scans enable continuous monitoring and change detection
• Comparing scan results over time reveals new hosts and services (network drift)

Proposed Approach

1. CIDR range parsing:

   • Support CIDR notation (10.0.0.0/8, 192.168.1.0/24)
   • Support IP ranges (192.168.1.1-254)
   • Support comma-separated targets and target files
   • Exclude list for skipping specific IPs (--exclude, --exclude-file)
   • Use ipaddress module from Python stdlib

2. Host discovery:

   • ICMP ping sweep before port scanning
   • ARP discovery for local network segments
   • TCP SYN ping for hosts that block ICMP
   • Configurable: skip discovery and scan all (--Pn equivalent)

3. Scan scheduling:

   • Cron-style schedule configuration
   • Persistent scan jobs with SQLite storage
   • Delta reporting: highlight changes between scans
   • CLI commands: schedule add, schedule list, schedule remove

4. Diff reporting:

   • Compare current scan against previous baseline
   • Report new hosts, removed hosts, changed services
   • Output as structured diff (JSON, text table)

Acceptance Criteria

• CIDR and range parsing with exclusion support
• Host discovery with multiple methods
• Scan scheduling with persistent job storage
• Diff reporting between consecutive scans