feat: optional per-device IP address tracking via periodic ARP scan

[mwpastore]

Summary

Adds an opt-in, per-device IP address tracking feature: when enabled, UpSnap periodically ARP-scans the local subnets of opted-in devices and updates a device's IP address whenever its MAC address shows up at a different IP. This keeps WOL and ping working for DHCP devices whose leases change.

• Per-device toggle (disabled by default): "Track IP address" in Device settings → General (track_ip bool on the devices collection).
• Global interval in Settings under Ping interval (track_ip_interval on settings_private, cron syntax, same validation UX as the ping interval). Empty = feature off.
• The sweep (registered in SetPingJobs, so it re-arms on settings changes): fetches devices with track_ip = true, computes each device's subnet from ip+netmask, keeps only subnets directly attached to the host (a host interface address lies inside the subnet — routed devices are never scanned), skips /32 and anything broader than /16, dedupes, runs one nmap -sn per subnet, and updates any opted-in device whose MAC now sits at a different IP. Saves use IgnoreUnchangedFields(true) so concurrent status writes from the ping/wake crons are never clobbered. No devices opted in or no local subnets → the tick is a no-op.

Related work

• [BUG] False positive online status in DHCP environments completely breaks scheduled WOL seriousm4x/UpSnap#1750 documented the motivating problem: in DHCP environments a device's stale IP breaks status tracking and scheduled WOL.
• Drop IP requirement and add dns hostnames seriousm4x/UpSnap#1753 is the umbrella issue for it, proposing optional IPs plus DNS hostnames.
• feature: Support use of FQDN resolution with DNS and mDNS seriousm4x/UpSnap#1760 is the open draft PR implementing that via DNS/mDNS (FQDN) resolution.

This PR is intended as complementary to seriousm4x#1760, not competing: name resolution covers devices that have a resolvable name (AD, avahi, dynamic DNS) and works across routed subnets, while ARP tracking covers devices with no name at all (headless/IoT boxes, plain DHCP networks without dynamic DNS) but only on directly attached subnets. A device answers to a MAC even when nothing answers for its name. The two features touch adjacent parts of the device form, so whichever lands second will need a small rebase — happy to be the one to rebase over seriousm4x#1760 if it merges first.

Design notes

• ARP scan, not broadcast ping: modern OSes ship with ICMP-echo-to-broadcast disabled, and passive ARP-table reads miss hosts the daemon hasn't talked to (arp_accept=0 ignores gratuitous ARP). A privileged nmap -sn on the local segment does the ARP probing directly and reports MACs in its XML output — the same mechanism the existing network scan feature relies on.
• No new privileges or dependencies: requires nmap + CAP_NET_RAW, exactly like the existing network scan. Docker images already ship both; host network mode is already required for WOL. The docker-compose capability comment and README are updated accordingly.
• Wake path untouched: tracking is periodic-only; WakeDevice/SendMagicPacket are unchanged.
• Linux-first: capability raising is build-tagged (scan_linux.go / scan_other.go), mirroring the existing ping/scan pattern.

Deliberate duplication / follow-up refactor opportunity

networking/scan.go + scan_linux.go intentionally duplicate the nmap exec + capability-raising code from pb/handlerscan_linux.go so that no existing files in backend/pb are touched by this PR. If desired, a natural follow-up (or squash into this PR) would collapse handlerscan_linux.go/handlerscan_other.go into a single cross-platform handler that calls the shared networking.NmapScan, removing ~120 lines of duplicated handler code. I have that refactor working and can push it as a separate commit/PR on request — kept it out of this one to keep the diff strictly feature-scoped.

Translations

The 4 new UI strings are added to en-US.json only; other locales fall back to English for these keys until translated. Happy to add translations for the other 22 locales if you'd prefer complete files.

Testing

• go build for darwin and linux/amd64, go vet, gofmt, and existing networking tests all pass.
• Booted the compiled backend against a fresh pb_data: both migrations apply and the new track_ip / track_ip_interval columns are created with correct defaults (off / empty).
• Frontend: pnpm build and eslint pass; svelte-check clean apart from a pre-existing error in paraglide's generated output.
• An adversarial code review pass flagged a lost-update race between this cron's saves and the ping cron's status saves (PocketBase Save() writes all columns); fixed via IgnoreUnchangedFields(true).
• Soak-tested in production: I deployed a beta build of this branch to my own UpSnap instance (bare-metal systemd install on a Proxmox guest). The migrations applied cleanly to the live database, and the tracker followed a real device through an actual IP round trip (.142 → .143 → .142, driven by netplan changes on the device) within one sweep interval each way. Running with --dev SQL logging confirmed the tracking save writes only the ip column while the ping cron's concurrent full-row status updates continued unharmed, and state survived service restarts.

AI disclosure

This feature was developed with Claude Code (Anthropic). The design decisions, scope choices, and review/testing direction were mine; I've reviewed the changes and stand behind them.

🤖 Generated with Claude Code