v0.5.0: Backup bugfixes + monitoring page with metrics store

- Fix "Helyi mentés" showing "–" after controller restart by synthesizing
  LastBackup from snapshot history and LastDBDump from dump files on disk
- New monitoring page (/monitoring) with system info, metrics charts, and
  container resource overview
- SQLite metrics store (modernc.org/sqlite, pure Go, no CGO) with 60s
  collection interval and 30-day auto-prune
- REST API endpoints: /api/metrics/system, /api/metrics/containers/summary,
  /api/metrics/containers/{name}, /api/metrics/sysinfo
- Chart.js 4.4.7 embedded locally for offline environments
- System info provider reads hostname, OS, kernel, CPU, uptime from /proc
- Docker compose updated with /etc/os-release host mount

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/docker-compose.yml

@@ -25,6 +25,8 @@ services:
       - ${HDD_PATH:-/mnt/hdd_placeholder}:${HDD_PATH:-/mnt/hdd_placeholder}:ro
       # Host /sys — for CPU temperature reading (read-only)
       - /sys:/host/sys:ro
+      # Host OS info — for monitoring page system info
+      - /etc/os-release:/host/etc/os-release:ro
     environment:
       - TZ=Europe/Budapest
     labels: