Add Veeam Backup & Replication collector

PowerShell script for the VBR server, run as a SYSTEM scheduled task.
Each run reports every Get-VBRJob job's next scheduled run and any
sessions from the lookback window (default 48h) not yet delivered in
their final state; running sessions are resent until they finish.

- Session result maps to success/warning/failed; non-Stopped states
  report as running; message combines the session reason, objects that
  didn't succeed, and warning/error lines from the session log
- Delivered sessions are tracked in a state file; failures are retried
  next run, and a rejected API key stops the run immediately
- Install-VeeamCollector.ps1 writes an ACL-restricted config, copies
  the script to Program Files and registers the task
- tests/MockVeeam.ps1 provides stand-in cmdlets for testing without VBR

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

README.md

@@ -12,6 +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)).
 
 ## Data model