refactor: extract monitoring surface into DomainViewModel+Monitoring (v5 step 4, 2/n)

Second slice of the DomainViewModel decomposition, stacked on the audit split.

- Moves the monitoring configuration surface — notification authorization, the
  settings mutators (enabled/scope/interval/adaptive/sensitivity/quiet-hours/
  alert filter/alerts/selection), toggleMonitoring, runMonitoringNow, the manual
  run, and the per-domain interval/status labels — plus the private
  intervalLabel helper into DomainViewModel+Monitoring.swift. Pure move.
- persistMonitoringSettings(...) and sanitizeMonitoringSelection() are promoted
  private -> internal: the tracked-domain lifecycle (add/delete/clear) on the
  main type calls them too, so they stay put and are now reachable cross-file.
  The two visibility drops are the only non-deletion edits to DomainViewModel.

DomainViewModel.swift: 4761 -> 4601 lines. App builds clean; unit suite 58/58.
No project.pbxproj change (DomainDig/ is a synchronized group).

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