### Check for existing issues

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

### The Feature

Rename the ten unprefixed Click `envvar=` arguments in `litellm/proxy/proxy_cli.py` to use the `LITELLM_*` prefix that the rest of the codebase uses. Keep the existing unprefixed names working as a non-breaking fallback via Click's list-of-envvars feature, and optionally emit a deprecation warning when an unprefixed name is the one that resolved.

Concretely, for `--host`:

```diff - "--host", default="0.0.0.0", help="Host for the server to listen on.", envvar="HOST" + "--host", default="0.0.0.0", help="Host for the server to listen on.", + envvar=["LITELLM_HOST", "HOST"] ```

Same shape for the other nine options listed below. Click resolves the first matched env var, so `LITELLM_HOST` takes precedence while `HOST` remains a working fallback for any existing deployment relying on it.

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## Version

Observed in `litellm v1.83.14`, file `litellm/proxy/proxy_cli.py`.

### Motivation, pitch

**LiteLLM already has a de-facto `LITELLM_*` env-var naming convention.** A grep across `litellm` 1.83.14 finds ~130 distinct `LITELLM_*` env vars r…