feat(api): add file listing endpoint

- Add GET /api endpoint to list all markdown files
- Filter to only include .md files
- Return JSON response with files array
- Add comprehensive tests for file listing functionality

backend/internal/server/server.go

@@ -31,6 +31,7 @@ func NewServer(host string, port int, handler http.Handler, log *logrus.Logger)
 
 func (s *Server) Start() error {
 	router := mux.NewRouter()
+	router.Handle("/api", s.handler).Methods("GET")
 	router.Handle("/api/{filename:.+.md}", s.handler)
 	router.PathPrefix("/").Handler(http.FileServer(http.Dir("frontend/dist")))