refactor: simplify architecture and harden lifecycle

Remove obsolete implementations, centralize background task ownership and terminal-state recovery, consolidate frontend routing and log streaming, and enforce project-wide verification in CI.

server/internal/agent/agent_test.go

@@ -0,0 +1,36 @@
+package agent
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"sync/atomic"
+	"testing"
+
+	"go.uber.org/zap"
+)
+
+func TestSubmitCommandResultRetriesWithCanceledCommandContext(t *testing.T) {
+	var requests atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		if requests.Add(1) < 3 {
+			http.Error(w, "temporarily unavailable", http.StatusServiceUnavailable)
+			return
+		}
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer server.Close()
+
+	agent := &Agent{
+		client: NewMasterClient(server.URL, "token", false),
+		logger: zap.NewNop(),
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	agent.submitCommandResult(ctx, 17, false, "backup failed", nil)
+
+	if got := requests.Load(); got != 3 {
+		t.Fatalf("submit requests = %d, want 3", got)
+	}
+}