fix: add missing package

Author: ulleo

backend/pyproject.toml

@@ -24,6 +24,7 @@ dependencies = [
     "langchain-openai>=0.3,<0.4",
     "langchain-community>=0.3,<0.4",
     "langchain-huggingface>=0.2.0",
+    "sentence-transformers>=4.0.2",
     "langgraph>=0.3,<0.4",
     "pgvector>=0.4.1",
     "dashscope>=1.14.0,<2.0.0",