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

internal/models/models.go

@@ -1,7 +1,8 @@
 package models
 
 type Credentials struct {
-	RefreshToken string `json:"refresh_token"`
+	RefreshToken string `json:"refresh_token,omitempty"`
+	APIToken     string `json:"api_token,omitempty"`
 }
 
 type TokenResponse struct {