Alarms — severity, escalation, Mission anomaly spawn

Alarms fire when a sensor reading crosses a threshold or a loop reports unhealthy.

Severities:
- **Critical** — life-threatening if not addressed inside the response window. Auto-spawns a missionAnomalies record in Mission. Pages the flight team.
- **Major** — degrades system performance; needs action this shift.
- **Minor** — informational; trend to watch.
- **Advisory** — out of nominal but not yet alarming.

The Alarms tab shows open alarms across the fleet, sorted critical-first. Each alarm has:
- Source sensor or loop
- Threshold crossed (e.g. "pCO2 > 0.8 kPa")
- Time triggered
- Vehicle
- Current value

To clear:
- Open the alarm → "Acknowledge" (records seen by whom) → take corrective action → "Clear" (logged with optional cause + notes)
- Critical alarms cannot be cleared without a clearing note (a sentence on what was done)

Severity-escalation trigger: an alarm at Major that hasn't been acknowledged for the configured window (default 30 min) auto-escalates to Critical, which fans out to Mission.