feat: default pinned models

Co-Authored-By: Classic298 <27028174+Classic298@users.noreply.github.com>

backend/open_webui/config.py

@@ -1141,6 +1141,12 @@ DEFAULT_MODELS = PersistentConfig(
     "DEFAULT_MODELS", "ui.default_models", os.environ.get("DEFAULT_MODELS", None)
 )
 
+DEFAULT_PINNED_MODELS = PersistentConfig(
+    "DEFAULT_PINNED_MODELS",
+    "ui.default_pinned_models",
+    os.environ.get("DEFAULT_PINNED_MODELS", None),
+)
+
 try:
     default_prompt_suggestions = json.loads(
         os.environ.get("DEFAULT_PROMPT_SUGGESTIONS", "[]")