feat: ntfy alerts for new attackers and port changes

[Jamkris]

Summary

Extends the background alert poller beyond ban spikes:

• New attacker IPs — notify when an attacker IP appears that wasn't seen last pass.
• Open-port changes — notify when listening ports are opened or closed.

Each is independently toggleable in Settings → Notifications; both default off. The poller now runs all three detectors each cycle, each priming on first pass so no alert fires for pre-existing state.

Test plan

• python -m pytest -q → 113 passed (6 new alerting tests).

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Summary by cubic

Adds ntfy alerts for new attacker IPs and open-port changes, extending the background poller beyond ban spikes. Helps catch fresh threats and unexpected service exposure.

• New Features
  • Toggle each alert in Settings; both default off.
  • First pass primes state so existing IPs/ports don’t alert.
  • Alert loop runs all detectors each cycle (ban spikes, new attackers, port changes).
  • Settings UI adds two checkboxes; i18n strings included.
  • Tests added for both detectors.

^{Written for commit a571911. Summary will update on new commits.}