feat(stats): resource metrics dashboard + sites logs/stats

Background collector samples CPU/memory/network/block I/O for every
instance and site on a configurable interval (default 15s, range
5-300s), persists samples to SQLite with a configurable retention
window (default 2h, range 0-24h), and skips ticks gracefully when
the Docker daemon is unreachable. Settings are reloadable without
a restart — each tick re-reads them.

New API endpoints:
- GET /api/system/stats (host snapshot: info + df)
- GET /api/system/stats/history
- GET /api/system/stats/top?by=cpu|memory
- GET /api/projects/{id}/stages/{s}/instances/{iid}/stats/history
- GET /api/sites/{id}/stats[/history]
- GET /api/sites/{id}/logs (SSE + JSON, reuses instance log streamer)

Frontend:
- ECharts added with tree-shaken imports (~180KB gzip) for
  future-proof time-series/gantt/graph visualizations
- CollapsibleSection wraps all dashboard sections (system health,
  daemons, system resources, static sites, projects) with
  localStorage-persisted open state
- SystemResourcesCard shows capacity tiles, workload utilization
  chart with 30m/2h/6h/24h window picker, disk breakdown with
  reclaimable callouts, and top 5 consumers
- ContainerStats and ContainerLogs take a source discriminated union
  so sites reuse the same components as instances; sites detail page
  embeds both for Deno backend debugging
- Settings › Maintenance exposes collection interval + retention
- Docker-unavailable state returns 503 and renders an amber banner
  instead of a generic 500

Full i18n coverage (en + ru) for all new strings.

web/src/routes/settings/maintenance/+page.svelte

@@ -21,6 +21,8 @@
 
 	let staleThresholdDays = $state('7');
 	let imagePruneThresholdMb = $state('1024');
+	let statsIntervalSeconds = $state('15');
+	let statsRetentionHours = $state('2');
 
 	async function load() {
 		loading = true;
@@ -28,6 +30,8 @@
 			const s = await getSettings();
 			staleThresholdDays = String(s.stale_threshold_days ?? 7);
 			imagePruneThresholdMb = String(s.image_prune_threshold_mb ?? 1024);
+			statsIntervalSeconds = String(s.stats_interval_seconds ?? 15);
+			statsRetentionHours = String(s.stats_retention_hours ?? 2);
 		} catch (err) {
 			toasts.error(err instanceof Error ? err.message : $t('settingsGeneral.loadFailed'));
 		} finally {
@@ -38,9 +42,20 @@
 	async function handleSave() {
 		saving = true;
 		try {
+			const intervalParsed = parseInt(statsIntervalSeconds, 10);
+			const retentionParsed = parseInt(statsRetentionHours, 10);
+			// Interval 0 disables collection; otherwise clamp to [5, 300].
+			const interval = isNaN(intervalParsed)
+				? 15
+				: intervalParsed === 0
+					? 0
+					: Math.max(5, Math.min(300, intervalParsed));
+			const retention = isNaN(retentionParsed) ? 2 : Math.max(0, Math.min(24, retentionParsed));
 			await updateSettings({
 				stale_threshold_days: Math.max(1, parseInt(staleThresholdDays, 10) || 7),
-				image_prune_threshold_mb: Math.max(0, parseInt(imagePruneThresholdMb, 10) || 0)
+				image_prune_threshold_mb: Math.max(0, parseInt(imagePruneThresholdMb, 10) || 0),
+				stats_interval_seconds: interval,
+				stats_retention_hours: retention
 			} as any);
 			toasts.success($t('settingsGeneral.saved'));
 		} catch (err) {
@@ -99,6 +114,22 @@
 					placeholder="1024"
 					helpText={$t('settings.pruneThresholdHelp')}
 				/>
+				<FormField
+					label={$t('statsSettings.intervalLabel')}
+					name="statsIntervalSeconds"
+					type="number"
+					bind:value={statsIntervalSeconds}
+					placeholder="15"
+					helpText={$t('statsSettings.intervalHelp')}
+				/>
+				<FormField
+					label={$t('statsSettings.retentionLabel')}
+					name="statsRetentionHours"
+					type="number"
+					bind:value={statsRetentionHours}
+					placeholder="2"
+					helpText={$t('statsSettings.retentionHelp')}
+				/>
 			</div>
 
 			<div class="mt-6">