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

README.md

@@ -26,9 +26,14 @@ go build -o canvasarchiver ./cmd/canvasarchiver
    ./canvasarchiver -fo
    ```
 
-   Or for videos-only mode (all Panopto videos flat, no files):
+    Or for videos-only mode (all Panopto videos flat, no files):
+    ```bash
+    ./canvasarchiver -vo
+    ```
+
+   Or to use a manually generated Canvas API token:
    ```bash
-   ./canvasarchiver -vo
+   ./canvasarchiver --api
    ```
 
 2. On first run, you'll be prompted to authenticate:
@@ -53,6 +58,7 @@ go build -o canvasarchiver ./cmd/canvasarchiver
 | `-vo` | Videos only — scan recordings and all module video items, download everything flat into one directory; skips regular files |
 | `-me` | Download all enrolled courses |
 | `-n` | Prefix modules with order numbers `[1]`, `[2]`, etc. |
+| `--api` | Use a manually generated Canvas API token instead of OAuth |
 
 
 ## Configuration
@@ -68,6 +74,8 @@ The following constants can be modified in `internal/config/config.go`:
 
 Credentials are stored in `credentials.json` after the first successful login. The refresh token is automatically used for subsequent runs.
 
+With `--api`, enter a manually generated Canvas API token on first use. It is saved to `credentials.json` and reused on later `--api` runs.
+
 ## Notes
 
 - Files are organized to match Canvas structure