fix: revert ExternalUrl auth to DownloadVideo, remove dead debug code

DownloadVideo already handles the Panopto Login.aspx canvas auth form
correctly through its multi-step form submission logic. The real fix for
ExternalUrl Panopto folder links was the URL normalization (List.aspx?folderID=
→ List.aspx#folderID="...") already in DownloadVideo.

Remove DownloadExternalPanoptoURL (dead code) and all debug prints.
Update README with -vo flag documentation.

README.md

@@ -21,11 +21,16 @@ go build -o canvasarchiver ./cmd/canvasarchiver
    ./canvasarchiver
    ```
 
-   Or for files-only mode:
+   Or for files-only mode (all files flat, no videos):
    ```bash
    ./canvasarchiver -fo
    ```
 
+   Or for videos-only mode (all Panopto videos flat, no files):
+   ```bash
+   ./canvasarchiver -vo
+   ```
+
 2. On first run, you'll be prompted to authenticate:
    - Visit the provided OAuth URL
    - Authorize the application
@@ -38,11 +43,14 @@ go build -o canvasarchiver ./cmd/canvasarchiver
    - Module content (to `Modules/`)
    - Panopto recordings (to `Recordings/`)
 
+   In `-vo` mode, only videos are downloaded — all into the course root directory (no subdirectories).
+
 ### Flags
 
 | Flag | Description |
 |------|-------------|
-| `-fo` | Files only mode - download all files to a single directory without module structure |
+| `-fo` | Files only — download all files flat into one directory; skips videos and module structure |
+| `-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. |