feat(litellm): add UI_USERNAME/UI_PASSWORD for the admin UI login

Without these, LiteLLM's /ui falls back to username 'admin' and the
master key as the password, forcing the master key to be typed into
a browser. generate-secrets.sh now fills UI_PASSWORD like the other
secrets.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q2CR8yawSf7pwAVYnjwFea

.env.example

@@ -28,3 +28,8 @@ LITELLM_PORT=4000
 LITELLM_MASTER_KEY=
 LITELLM_SALT_KEY=
 LITELLM_DB_PASSWORD=
+# Admin UI login (https://<proxy>/ui). Without these, LiteLLM falls back to
+# username "admin" / password = LITELLM_MASTER_KEY — set these instead so the
+# master key never has to be typed into the browser.
+UI_USERNAME=admin
+UI_PASSWORD=