Pin langchain dependencies to 0.2.x for compatibility

langchain 1.x removed langchain.chains module, breaking the
ConversationalRetrievalChain import. Pinning to 0.2.x which
still supports the legacy chains API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

requirements.txt

@@ -1,8 +1,8 @@
-langchain>=0.1.0
-langchain-openai>=0.0.2
-langchain-community>=0.0.16
-langchain-chroma>=0.0.5
-chromadb>=0.4.22
+langchain>=0.2.0,<0.3.0
+langchain-openai>=0.1.0,<0.3.0
+langchain-community>=0.2.0,<0.3.0
+langchain-chroma>=0.1.0,<0.3.0
+chromadb>=0.4.22,<0.6.0
 streamlit==1.41.0
 python-dotenv==1.0.1
 huggingface-hub>=0.19.4
@@ -12,4 +12,4 @@ flake8>=6.0.0
 numpy>=1.24.0
 chardet>=5.0.0
 tenacity>=8.2.3
-datasets==3.2.0
+datasets==3.2.0