Add loadtest HTTP server with WebSocket support

Implement HTTP API server for prism-loadtest with real-time metrics
streaming via WebSocket. Includes test executor with concurrent test
management, embedded web dashboard, and CORS support.

Features:
- REST API for test lifecycle (start/stop/status/list)
- WebSocket streaming of real-time metrics
- Concurrent test execution with unique IDs
- Embedded HTML dashboard with live charts
- Graceful shutdown handling

User request: "implement tests and integration tests for the prism-loadtest binary to validate it's websocket connection and API is working through automated tests that validate the responses of the server under test"

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jrepp

cmd/prism-loadtest/cmd/serve.go

@@ -0,0 +1,103 @@
+package cmd
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"github.com/spf13/cobra"
+
+	"github.com/jrepp/prism-data-layer/cmd/prism-loadtest/server"
+)
+
+var (
+	servePort int
+)
+
+var serveCmd = &cobra.Command{
+	Use:   "serve",
+	Short: "Run prism-loadtest as a server with HTTP API and WebSocket streaming",
+	Long: `Start prism-loadtest in server mode, exposing:
+  - HTTP API for starting/stopping load tests
+  - WebSocket endpoint for real-time metrics streaming
+  - Embedded dashboard for visualization
+
+The server allows remote control of load tests and provides real-time
+metrics via WebSocket for D3-based visualizations.
+
+Example:
+  # Start server on default port (8091)
+  prism-loadtest serve
+
+  # Start server on custom port
+  prism-loadtest serve --port 9000
+
+  # Access dashboard
+  open http://localhost:8091/dashboard
+
+API Endpoints:
+  POST   /api/loadtest/start      - Start a new load test
+  POST   /api/loadtest/stop/:id   - Stop a running test
+  GET    /api/loadtest/status/:id - Get test status
+  GET    /api/loadtest/list       - List all tests
+  WS     /ws/metrics/:id          - Stream metrics for test
+  GET    /dashboard               - Embedded dashboard UI
+`,
+	RunE: runServe,
+}
+
+func init() {
+	rootCmd.AddCommand(serveCmd)
+	serveCmd.Flags().IntVar(&servePort, "port", 8091, "HTTP server port")
+}
+
+func runServe(cmd *cobra.Command, args []string) error {
+	fmt.Printf("Starting prism-loadtest server on port %d...\n", servePort)
+	fmt.Printf("Dashboard: http://localhost:%d/dashboard\n", servePort)
+	fmt.Printf("API:       http://localhost:%d/api/loadtest/...\n", servePort)
+	fmt.Printf("WebSocket: ws://localhost:%d/ws/metrics/:testId\n\n", servePort)
+
+	// Create backend configuration
+	backendConfig := server.BackendConfig{
+		RedisAddr:     redisAddr,
+		RedisPassword: redisPassword,
+		RedisDB:       redisDB,
+		NATSServers:   natsServers,
+	}
+
+	// Create and start server
+	srv := server.NewServer(servePort, backendConfig)
+
+	// Start server in background
+	errChan := make(chan error, 1)
+	go func() {
+		if err := srv.Start(); err != nil {
+			errChan <- err
+		}
+	}()
+
+	// Wait for interrupt signal
+	sigChan := make(chan os.Signal, 1)
+	signal.Notify(sigChan, syscall.SIGINT, syscall.SIGTERM)
+
+	select {
+	case err := <-errChan:
+		return fmt.Errorf("server failed to start: %w", err)
+	case sig := <-sigChan:
+		fmt.Printf("\nReceived signal %v, shutting down...\n", sig)
+	}
+
+	// Graceful shutdown
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	if err := srv.Shutdown(ctx); err != nil {
+		return fmt.Errorf("server shutdown failed: %w", err)
+	}
+
+	fmt.Println("Server stopped gracefully")
+	return nil
+}