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.

internal/store/models.go

@@ -78,9 +78,40 @@ type Settings struct {
 	BackupEnabled       bool   `json:"backup_enabled"`
 	BackupIntervalHours  int    `json:"backup_interval_hours"`
 	BackupRetentionCount int    `json:"backup_retention_count"`
+	StatsIntervalSeconds int    `json:"stats_interval_seconds"` // 0 disables collection
+	StatsRetentionHours  int    `json:"stats_retention_hours"`  // 0 disables collection
 	UpdatedAt            string `json:"updated_at"`
 }
 
+// ContainerStatsSample is one persisted sample of container resource usage.
+// Cumulative counters (network, block I/O) require differencing two samples
+// to get rates; CPU is already a percent-since-previous-sample value.
+type ContainerStatsSample struct {
+	ContainerID     string `json:"container_id"`
+	OwnerType       string `json:"owner_type"` // "instance" or "site"
+	OwnerID         string `json:"owner_id"`
+	TS              int64  `json:"ts"` // Unix seconds UTC
+	CPUPercent      float64 `json:"cpu_percent"`
+	MemoryUsage     int64  `json:"memory_usage"`
+	MemoryLimit     int64  `json:"memory_limit"`
+	NetworkRxBytes  int64  `json:"network_rx_bytes"`
+	NetworkTxBytes  int64  `json:"network_tx_bytes"`
+	BlockReadBytes  int64  `json:"block_read_bytes"`
+	BlockWriteBytes int64  `json:"block_write_bytes"`
+}
+
+// SystemStatsSample is one persisted host-level snapshot that aggregates
+// workload usage across all containers plus daemon capacity + disk totals.
+type SystemStatsSample struct {
+	TS                 int64   `json:"ts"` // Unix seconds UTC
+	NCPU               int     `json:"ncpu"`
+	MemoryTotal        int64   `json:"memory_total"`
+	WorkloadCPUPercent float64 `json:"workload_cpu_percent"`
+	WorkloadMemUsage   int64   `json:"workload_mem_usage"`
+	ContainersRunning  int     `json:"containers_running"`
+	DiskTotalBytes     int64   `json:"disk_total_bytes"`
+}
+
 // Backup represents a backup metadata record.
 type Backup struct {
 	ID         string `json:"id"`