feat(server): add IP access control via --allow-ips / --deny-ips

[HuRuilizhen]

Scope

• Add --allow-ips and --deny-ips CLI options supporting comma-separated
  IPs / CIDR notation (e.g., 192.168.1.0/24)
• Enforce access rules in the file download handler:
  • deny‑list match → 403 Forbidden
  • allow‑list present with no match → 403 Forbidden
  • no rules → allow all (current default)
• Surface active rules in the startup banner for easy verification

Implementation

• New CLI flags parsed with clap::value_parser!(IpNet)
• ipnet crate for CIDR matching
• Client IP extracted via axum::extract::ConnectInfo<SocketAddr>
• AppState::is_ip_allowed() encapsulates the allow/deny logic

Testing

• cargo check
• cargo clippy --all-targets --all-features
• cargo fmt

Notes

• This PR does not change existing file-serving behavior when no IP
  rules are provided
• The startup banner displays Allow IPs / Deny IPs only when
  the corresponding lists are non‑empty

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[HuRuilizhen]

Enforce IP filter on all file-discovery endpoints

Addressed in a454f77.

Applied the same is_ip_allowed() gate to both index_page (/) and list_files (/files) so that the IP ACL covers all public endpoints consistently. Blocked clients now receive 403 Forbidden on any endpoint, eliminating the information-disclosure bypass.