fix: cache headers and frontend pro tier resolution

routes/legacy_routes.py

@@ -225,7 +225,11 @@ async def get_available_providers():
             "model": model,
         })
 
-    return {"providers": available}
+    return JSONResponse(
+        status_code=200,
+        headers={"Cache-Control": "no-cache, no-store, must-revalidate"},
+        content={"providers": available}
+    )
 
 
 @router.get("/languages")