refactor: extract core server package with handlers, routes, and lifecycle

Created server package with:
- server.go: Server struct and core methods
- handlers.go: HTTP handlers for health, KV operations, cluster management
- routes.go: HTTP route setup
- lifecycle.go: Server startup/shutdown logic

This moves ~400 lines of server-related code from main.go to dedicated
server package for better organization.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

server/lifecycle.go

@@ -0,0 +1,94 @@
+package server
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/sirupsen/logrus"
+)
+
+// Start the server and initialize all services
+func (s *Server) Start() error {
+	router := s.setupRoutes()
+
+	addr := fmt.Sprintf("%s:%d", s.config.BindAddress, s.config.Port)
+	s.httpServer = &http.Server{
+		Addr:    addr,
+		Handler: router,
+	}
+
+	s.logger.WithFields(logrus.Fields{
+		"node_id": s.config.NodeID,
+		"address": addr,
+	}).Info("Starting KVS server")
+
+	// Start gossip and sync routines
+	s.startBackgroundTasks()
+
+	// Try to join cluster if seed nodes are configured and clustering is enabled
+	if s.config.ClusteringEnabled && len(s.config.SeedNodes) > 0 {
+		go s.bootstrap()
+	}
+
+	return s.httpServer.ListenAndServe()
+}
+
+// Stop the server gracefully
+func (s *Server) Stop() error {
+	s.logger.Info("Shutting down KVS server")
+
+	// Stop cluster services
+	s.gossipService.Stop()
+	s.syncService.Stop()
+
+	// Close storage services
+	if s.storageService != nil {
+		s.storageService.Close()
+	}
+
+	s.cancel()
+	s.wg.Wait()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	if err := s.httpServer.Shutdown(ctx); err != nil {
+		s.logger.WithError(err).Error("HTTP server shutdown error")
+	}
+
+	if err := s.db.Close(); err != nil {
+		s.logger.WithError(err).Error("BadgerDB close error")
+	}
+
+	return nil
+}
+
+// startBackgroundTasks initializes and starts cluster services
+func (s *Server) startBackgroundTasks() {
+	// Start cluster services
+	s.gossipService.Start()
+	s.syncService.Start()
+}
+
+// bootstrap joins cluster using seed nodes via bootstrap service
+func (s *Server) bootstrap() {
+	if len(s.config.SeedNodes) == 0 {
+		s.logger.Info("No seed nodes configured, running as standalone")
+		return
+	}
+
+	s.logger.Info("Starting bootstrap process")
+	s.setMode("syncing")
+
+	// Use bootstrap service to join cluster
+	if err := s.bootstrapService.JoinCluster(); err != nil {
+		s.logger.WithError(err).Error("Failed to join cluster")
+		s.setMode("normal")
+		return
+	}
+
+	s.setMode("normal")
+	s.logger.Info("Successfully joined cluster")
+}