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/web/templates/style.css

@@ -1477,6 +1477,125 @@ a.stat-card:hover {
     font-size: .8rem;
 }
 
+/* --- Monitoring page --- */
+.monitor-card {
+    background: var(--bg-card);
+    border-radius: var(--radius);
+    padding: 1.25rem;
+    border: 1px solid var(--border-color);
+    margin-bottom: 1.5rem;
+}
+.monitor-card h3 {
+    margin-bottom: .75rem;
+}
+.monitor-card-header {
+    display: flex;
+    align-items: center;
+    gap: 1rem;
+    margin-bottom: 1rem;
+    flex-wrap: wrap;
+}
+.monitor-card-header h3 {
+    margin-bottom: 0;
+    flex: 1;
+    min-width: 150px;
+}
+.time-range-bar {
+    display: flex;
+    gap: .35rem;
+}
+.sysinfo-grid {
+    display: grid;
+    grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
+    gap: .5rem;
+}
+.sysinfo-row {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    padding: .35rem .5rem;
+    border-bottom: 1px solid rgba(48, 54, 61, 0.3);
+    font-size: .9rem;
+}
+.sysinfo-row:last-child { border-bottom: none; }
+.sysinfo-label {
+    color: var(--text-secondary);
+    font-weight: 500;
+}
+.sysinfo-value {
+    color: var(--text-primary);
+    font-family: 'JetBrains Mono', monospace;
+    font-size: .85rem;
+}
+.charts-grid {
+    display: grid;
+    grid-template-columns: 1fr 1fr;
+    gap: 1rem;
+}
+.charts-grid-2 {
+    grid-template-columns: 1fr 1fr;
+}
+.chart-box {
+    background: var(--bg-secondary);
+    border-radius: 8px;
+    padding: .75rem;
+    border: 1px solid rgba(48, 54, 61, 0.5);
+}
+.chart-box-half {
+    flex: 1;
+    min-width: 0;
+}
+.chart-title {
+    font-size: .8rem;
+    font-weight: 600;
+    color: var(--text-secondary);
+    margin-bottom: .5rem;
+    text-transform: uppercase;
+    letter-spacing: .3px;
+}
+.chart-wrap {
+    position: relative;
+    height: 180px;
+}
+.chart-wrap-bar {
+    position: relative;
+    height: 250px;
+}
+.container-charts-row {
+    display: flex;
+    gap: 1rem;
+}
+.chart-empty {
+    text-align: center;
+    padding: 2rem;
+    color: var(--text-muted);
+    font-size: .9rem;
+}
+.storage-bars {
+    display: flex;
+    flex-direction: column;
+    gap: 1rem;
+}
+.storage-item {
+    padding: .25rem 0;
+}
+.storage-header {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    margin-bottom: .4rem;
+}
+.storage-label {
+    font-size: .85rem;
+    font-weight: 600;
+    color: var(--text-secondary);
+}
+.storage-value {
+    font-size: .8rem;
+    color: var(--text-muted);
+    font-family: 'JetBrains Mono', monospace;
+}
+
 /* Responsive */
 @media(max-width: 768px) {
     .sidebar { width: 100%; height: auto; position: relative; border-right: none; border-bottom: 1px solid var(--border-color); }
@@ -1491,4 +1610,7 @@ a.stat-card:hover {
     .stats-grid { grid-template-columns: repeat(3, 1fr); }
     .deploy-info { flex-direction: column; }
     .system-info-items { flex-direction: column; gap: 1rem; }
+    .charts-grid { grid-template-columns: 1fr; }
+    .container-charts-row { flex-direction: column; }
+    .sysinfo-grid { grid-template-columns: 1fr; }
 }