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/internal/backup/backup.go

@@ -439,6 +439,40 @@ func (m *Manager) GetFullStatus(nextDBDump, nextBackup time.Time) *FullBackupSta
 		for i, j := 0, len(m.cachedStatus.SnapshotHistory)-1; i < j; i, j = i+1, j-1 {
 			m.cachedStatus.SnapshotHistory[i], m.cachedStatus.SnapshotHistory[j] = m.cachedStatus.SnapshotHistory[j], m.cachedStatus.SnapshotHistory[i]
 		}
+
+		// Synthesize LastBackup from snapshot history if not in memory (e.g., after restart)
+		if m.cachedStatus.LastBackup == nil && len(m.cachedStatus.SnapshotHistory) > 0 {
+			latest := m.cachedStatus.SnapshotHistory[0] // already reversed, newest first
+			m.cachedStatus.LastBackup = &BackupStatus{
+				LastRun: latest.Time,
+				Success: latest.Success,
+				Snapshot: &SnapshotResult{
+					SnapshotID: latest.SnapshotID,
+				},
+			}
+		}
+
+		// Synthesize LastDBDump from DumpFiles on disk if not in memory
+		if m.cachedStatus.LastDBDump == nil && len(m.cachedStatus.DumpFiles) > 0 {
+			var results []DumpResult
+			var latestTime time.Time
+			for _, f := range m.cachedStatus.DumpFiles {
+				results = append(results, DumpResult{
+					DB:       DiscoveredDB{StackName: f.StackName, DBType: f.DBType, ContainerName: f.StackName},
+					FilePath: f.FileName,
+					Size:     f.Size,
+				})
+				if f.ModTime.After(latestTime) {
+					latestTime = f.ModTime
+				}
+			}
+			m.cachedStatus.LastDBDump = &DBDumpStatus{
+				LastRun: latestTime,
+				Results: results,
+				Success: true,
+			}
+		}
+
 		return m.cachedStatus
 	}