Fix openai/httpx proxies incompatibility on HuggingFace

openai==1.51.0 passes proxies= to httpx which httpx>=0.28 dropped.
Unpinning to openai>=1.55.0 which removed that argument.

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

requirements.txt

@@ -1,5 +1,5 @@
 streamlit==1.39.0
-openai==1.51.0
+openai>=1.55.0
 pymupdf==1.24.10
 pytesseract==0.3.13
 Pillow==10.4.0