Add Azure Backup (MARS agent) collector

PowerShell script for servers running the MARS agent, run as a SYSTEM
scheduled task (hourly) under Windows PowerShell 5.1, which the
agent's MSOnlineBackup module targets. Each server is one dashboard job.

- Backup jobs from Get-OBJob (in progress and -Previous) within the
  lookback window; restores are ignored. Completed with failed bytes
  or an error reports as warning
- Next run computed from the backup and System State policy schedules;
  left empty for every-N-weeks schedules to avoid false Overdue alarms
- Property reads tolerate unknown names, since the agent's object
  model isn't documented; Get-MarsCollectorDiagnostics.ps1 dumps the
  real structure for checking
- Install-MarsCollector.ps1 mirrors the Veeam installer; tests/MockMars.ps1
  provides stand-in cmdlets

Not yet run against a real MARS agent.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

@@ -12,7 +12,7 @@ browser ──nginx (basic auth, TLS)──▶ /api/* ─▶│ SQLite       │
 - `public/`: static frontend (no build step). Add `?mock=1` to the URL to use built-in sample data.
 - `backend/`: FastAPI API backed by a single SQLite file.
 - `deploy/`: nginx site and systemd unit.
-- `collectors/`: scripts that report results from backup products ([Veeam B&R](collectors/veeam/README.md)).
+- `collectors/`: scripts that report results from backup products ([Veeam B&R](collectors/veeam/README.md), [Azure Backup / MARS](collectors/mars/README.md)).
 
 ## Data model