feat: add admin endpoint to send digest emails for users

- Implemented a new endpoint `/admin/test/send-digest` in `admin_api.py` to trigger digest emails for specific users.
- Added a script `send_digest.py` to facilitate sending digest emails via the admin API.
- Enhanced the digest action handling in `digest_action_api.py` to support token peeking without consuming it.
- Updated the `send_digests` function in `digest_scheduler.py` to utilize the new `send_digest_for_user` function for sending emails.
- Introduced a new utility function `peek_token` in `digest_token.py` to validate tokens without consuming them.
- Modified the `ParentView.vue` component to handle digest actions upon receiving a digest token in the URL.
- Updated `.gitignore` to exclude sensitive certificate files.

.gitignore

@@ -8,3 +8,5 @@ backend/test-results/
 .vscode/keybindings.json
 .DS_Store
 **/.DS_Store
+frontend/vue-app/cert.pem
+frontend/vue-app/key.pem