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/lib/components/InstanceCard.svelte

@@ -147,15 +147,13 @@
 	</div>
 
 	{#if instance.status === 'running'}
-		<ContainerStats projectId={projectId} stageId={instance.stage_id} instanceId={instance.id} />
+		<ContainerStats source={{ kind: 'instance', projectId, stageId: instance.stage_id, instanceId: instance.id }} />
 	{/if}
 
 	{#if showLogs}
 		<div class="mt-2">
 			<ContainerLogs
-				{projectId}
-				stageId={instance.stage_id}
-				instanceId={instance.id}
+				source={{ kind: 'instance', projectId, stageId: instance.stage_id, instanceId: instance.id }}
 				onclose={() => { showLogs = false; }}
 			/>
 		</div>