Spoken audio cues for Atemschutz (Druckabfrage fällig + replace Rückzugsalarm siren with speech)

[DeepDiver1975]

Split out from #70 (which covers the ILS reminder audio and lands the cross-platform
one-shot playback groundwork).

Add spoken audio cues for the Atemschutz side, reusing the cross-platform one-shot
IAlarmService.Play(AlarmSound) path introduced in #70:

• "Druckabfrage fällig" — spoken cue when a Trupp's Atemschutzkontrolle (pressure
  check) becomes due; once per control cycle, re-armed after a recorded pressure.
• Replace the Rückzugsalarm siren with speech — drop the looping alarm.wav
  siren and announce "Rückzugsalarm", re-announced on a cadence (~15 s) until
  acknowledged (to preserve the siren's "insistent until handled" property).

Work:

• Extend the AlarmSound enum with PressureCheckDue and RetreatAlarm.
• Add the two spoken WAV assets (AI-generated): voice-druckabfrage.wav,
  voice-rueckzugsalarm.wav.
• Migrate ScbaViewModel.UpdateAlarm off Start()/Stop() onto Play(...), add the
  pressure-check-due edge-trigger, and rework the affected ScbaViewModelTests
  (IsSounding assertions -> spoken-cue assertions).
• Once the siren is gone, remove Start()/Stop() and alarm.wav.