fix(backend): resolve pytest dependency conflict and remove sentence-transformers

- Change pytest==8.0.0 to pytest==7.4.4 (conflict with pytest-asyncio<8)

- Remove sentence-transformers (not used with API-based embeddings)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/requirements.txt

@@ -3,12 +3,11 @@ uvicorn[standard]==0.27.0
 pydantic==2.5.3
 pydantic-settings==2.1.0
 chromadb==0.4.22
-sentence-transformers==2.3.1
 python-docx==1.1.0
 pypdf==4.0.2
 python-dotenv==1.0.0
 httpx==0.26.0
-pytest==8.0.0
+pytest==7.4.4
 pytest-asyncio==0.23.4
 tiktoken==0.5.2
 python-multipart==0.0.6