Add missing httpx dependency for Authlib's Starlette OAuth client

Deployed login crashed with ModuleNotFoundError: httpx, since Authlib's
starlette_client integration imports httpx for its async HTTP client but
it wasn't in requirements.txt. Verified by hitting the real /auth/login
route end-to-end and confirming it now 302s to Google's consent screen
instead of 500ing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

requirements.txt

@@ -4,4 +4,5 @@ streamlit>=1.42
 gspread
 google-auth
 Authlib
+httpx
 tomli_w