feat: add --api flag for Canvas API token authentication

- Add APIToken field to Credentials struct with omitempty JSON tag
- Implement GetAPIToken() method with persistent token storage
- Add SaveRefreshToken() helper to preserve API tokens during OAuth
- Implement --api CLI flag to use API token instead of OAuth
- Validate empty refresh tokens and skip refresh if invalid
- Update README with API token usage documentation

cmd/canvasarchiver/main.go

@@ -17,12 +17,19 @@ func main() {
 	videosOnly := flag.Bool("vo", false, "Videos only mode - download only Panopto videos to a single directory")
 	me := flag.Bool("me", false, "Download all enrolled courses")
 	moduleNumbers := flag.Bool("n", false, "Prefix modules with order numbers [1], [2], etc.")
+	apiToken := flag.Bool("api", false, "Use a manually generated Canvas API token instead of OAuth")
 	flag.Parse()
 
 	httpClient := &http.Client{}
 
 	authenticator := auth.NewAuthenticator(httpClient)
-	accessToken, err := authenticator.GetAccessToken()
+	var accessToken string
+	var err error
+	if *apiToken {
+		accessToken, err = authenticator.GetAPIToken()
+	} else {
+		accessToken, err = authenticator.GetAccessToken()
+	}
 	if err != nil {
 		fmt.Printf("Authentication failed: %v\n", err)
 		return