### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

Closest existing issues are #20499 ("no invite mails sent" - different symptom) and the closed enterprise / `/invitation/new` link-id fixes #12130 / #11978 / #15375. None cover the legacy OSS SMTP template.

### What happened?

On a self-hosted **OSS deployment (no enterprise license)**, creating an internal user with `send_invite_email: true` sends the invitation email fine, but the **"Get Started here" button links to the bare `PROXY_BASE_URL`** - no invitation id/hash. The link lands on the login page; onboarding can never complete because `GET /onboarding/get_token` requires the invitation id in the URL, and it's never there. The invited user cannot join.

**Root cause (source-verified @ `v1.93.0`):** the legacy email path builds the button from `PROXY_BASE_URL` only.

- `litellm/integrations/email_templates/templates.py` - `USER_INVITED_EMAIL_TEMPLATE`:

```html <a href="{base_url}" ...>Get Started here</a> ```

- `litellm/integrations/SlackAlerting/slack_alerting.py` - `send_key_created_or_user_invited_email` formats it with `base_url=base_url, team_na…