feat(observability): phase 8 - container stats, notifications & dashboard

Add container monitoring and notification system:
- Docker Stats API: real-time CPU/memory for running containers
- Webhook notifications for errors (deploy failures, stale, proxy unhealthy)
- Event log auto-pruning (daily, 30-day retention)
- ContainerStats component with auto-polling progress bars
- SystemHealthCard dashboard widget with running/proxy/error counts
- Full EN/RU i18n for stats and system health

cmd/server/main.go

@@ -12,6 +12,8 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/robfig/cron/v3"
+
 	dockerwatcher "github.com/alexei/docker-watcher"
 	"github.com/alexei/docker-watcher/internal/api"
 	"github.com/alexei/docker-watcher/internal/auth"
@@ -145,6 +147,51 @@ func main() {
 		slog.Warn("failed to start proxy health monitor", "error", err)
 	}
 
+	// Start daily event log pruning cron job.
+	cronScheduler := cron.New()
+	if _, err := cronScheduler.AddFunc("@daily", func() {
+		pruned, err := db.PruneEvents(30)
+		if err != nil {
+			slog.Error("event log prune failed", "error", err)
+			return
+		}
+		if pruned > 0 {
+			slog.Info("pruned old event log entries", "count", pruned)
+		}
+	}); err != nil {
+		slog.Warn("failed to schedule event prune cron", "error", err)
+	}
+	cronScheduler.Start()
+
+	// Subscribe to error events and forward notifications.
+	notifySub := eventBus.Subscribe(func(evt events.Event) bool {
+		if evt.Type != events.EventLog {
+			return false
+		}
+		p, ok := evt.Payload.(events.EventLogPayload)
+		if !ok {
+			return false
+		}
+		return p.Severity == "error"
+	})
+	go func() {
+		for evt := range notifySub {
+			p, ok := evt.Payload.(events.EventLogPayload)
+			if !ok {
+				continue
+			}
+			currentSettings, err := db.GetSettings()
+			if err != nil || currentSettings.NotificationURL == "" {
+				continue
+			}
+			notifier.Send(currentSettings.NotificationURL, notify.Event{
+				Type:    p.Source + "_error",
+				Project: p.Source,
+				Error:   p.Message,
+			})
+		}
+	}()
+
 	// Build API server.
 	apiServer := api.NewServer(db, dockerClient, npmClient, dep, webhookHandler, eventBus, encKey)
 	apiServer.SetStaleScanner(staleScanner)
@@ -190,6 +237,8 @@ func main() {
 	slog.Info("shutting down...")
 
 	// Stop accepting new work.
+	cronScheduler.Stop()
+	eventBus.Unsubscribe(notifySub)
 	proxyHealth.Stop()
 	staleScanner.Stop()
 	poller.Stop()