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

internal/api/router.go

@@ -136,6 +136,7 @@ func (s *Server) Router() chi.Router {
 				r.Get("/", s.getProject)
 				r.Get("/stages/{stage}/env", s.listStageEnv)
 				r.Get("/stages/{stage}/instances", s.listInstances)
+				r.Get("/stages/{stage}/instances/{iid}/stats", s.getInstanceStats)
 				r.Get("/volumes", s.listVolumes)
 			})
 			r.Get("/deploys", s.listDeploys)

internal/api/stats.go

@@ -0,0 +1,39 @@
+package api
+
+import (
+	"errors"
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+
+	"github.com/alexei/docker-watcher/internal/store"
+)
+
+// getInstanceStats handles GET /api/projects/{id}/stages/{stage}/instances/{iid}/stats.
+// Returns CPU and memory stats for the container backing the given instance.
+func (s *Server) getInstanceStats(w http.ResponseWriter, r *http.Request) {
+	instanceID := chi.URLParam(r, "iid")
+
+	inst, err := s.store.GetInstanceByID(instanceID)
+	if err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			respondNotFound(w, "instance")
+			return
+		}
+		respondError(w, http.StatusInternalServerError, "failed to get instance: "+err.Error())
+		return
+	}
+
+	if inst.ContainerID == "" {
+		respondError(w, http.StatusBadRequest, "instance has no container")
+		return
+	}
+
+	stats, err := s.docker.GetContainerStats(r.Context(), inst.ContainerID)
+	if err != nil {
+		respondError(w, http.StatusInternalServerError, "failed to get container stats: "+err.Error())
+		return
+	}
+
+	respondJSON(w, http.StatusOK, stats)
+}