feat: database layer + canvas validator/converter + LLM inference activities

- Add pkg/db models and CRUD methods for workflows
- Add internal/routing canvas validator (DAG check, connectivity)
- Add internal/routing canvas converter (Canvas → WorkflowSpec)
- Register LLMInferenceActivity and LLMBatchInferenceActivity
- Update api/server and cmd/server with database integration
- Add K8s environment variable support
- Update activity knowledge base with LLM activities
- Add .env.example configuration template

internal/api/server.go

@@ -0,0 +1,101 @@
+package api
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+	"strings"
+
+	"go.temporal.io/sdk/client"
+
+	"github.com/rockliang/poimen/workflows/pkg/db"
+)
+
+// Server handles HTTP routing for workflow APIs
+type Server struct {
+	api    *WorkflowAPI
+	logger *log.Logger
+}
+
+// NewServer creates new HTTP server with database connection
+func NewServer(database *db.DB, temporalClient client.Client, logger *log.Logger) *Server {
+	return &Server{
+		api:    NewWorkflowAPI(database, temporalClient, logger),
+		logger: logger,
+	}
+}
+
+// ServeHTTP dispatches HTTP requests to appropriate handler
+func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	// Enable CORS
+	w.Header().Set("Access-Control-Allow-Origin", "*")
+	w.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
+	w.Header().Set("Access-Control-Allow-Headers", "Content-Type, Authorization")
+
+	if r.Method == http.MethodOptions {
+		w.WriteHeader(http.StatusOK)
+		return
+	}
+
+	path := r.URL.Path
+	method := r.Method
+
+	s.logger.Printf("%s %s", method, path)
+
+	// Route requests
+	switch {
+	// Workflow endpoints
+	case path == "/workflows" && method == http.MethodPost:
+		s.api.CreateWorkflow(w, r)
+	case path == "/workflows" && method == http.MethodGet:
+		s.api.ListWorkflows(w, r)
+	case strings.HasPrefix(path, "/workflows/") && method == http.MethodGet:
+		id := strings.TrimPrefix(path, "/workflows/")
+		// Exclude special paths
+		if !strings.Contains(id, "/") {
+			s.api.GetWorkflow(w, r, id)
+		} else if strings.HasSuffix(id, "/executions") {
+			// GET /workflows/{id}/executions
+			workflowID := strings.TrimSuffix(id, "/executions")
+			s.api.ListExecutions(w, r, workflowID)
+		}
+	case strings.HasPrefix(path, "/workflows/") && method == http.MethodPut:
+		id := extractID(path, "/workflows/")
+		s.api.UpdateWorkflow(w, r, id)
+	case strings.HasPrefix(path, "/workflows/") && method == http.MethodDelete:
+		id := extractID(path, "/workflows/")
+		s.api.DeleteWorkflow(w, r, id)
+
+	// Execute workflow
+	case strings.HasSuffix(path, "/execute") && method == http.MethodPost:
+		// POST /workflows/{id}/execute
+		parts := strings.Split(path, "/")
+		if len(parts) >= 4 && parts[1] == "workflows" && parts[3] == "execute" {
+			s.api.ExecuteWorkflow(w, r, parts[2])
+		}
+
+	// Execution endpoints
+	case strings.HasPrefix(path, "/executions/") && method == http.MethodGet:
+		id := extractID(path, "/executions/")
+		s.api.GetExecution(w, r, id)
+
+	default:
+		http.Error(w, "Not found", http.StatusNotFound)
+	}
+}
+
+// extractID extracts resource ID from path
+func extractID(path, prefix string) string {
+	id := strings.TrimPrefix(path, prefix)
+	if idx := strings.Index(id, "/"); idx != -1 {
+		return id[:idx]
+	}
+	return id
+}
+
+// Start starts the HTTP server
+func (s *Server) Start(port int) error {
+	addr := fmt.Sprintf(":%d", port)
+	s.logger.Printf("Starting API server on %s", addr)
+	return http.ListenAndServe(addr, s)
+}