fix(oauth): use gr.OAuthToken for hf_auth + remove hf_oauth from README to resolve secret collision and Gradio type-hint ValueError

Fixes Gradio 6.14 startup ValueError on @app .api endpoints.

ROOT CAUSE
- Previous version of handle_hf_auth used `request: gr.Request = None`.
- With `from __future__ import annotations`, Gradio's get_type_hints()
must resolve the annotation. While gr.Request does resolve, Gradio's
stricter type-check in 6.14 raises `ValueError: API endpoints must
have type hints` for some annotation patterns at launch time.

FIX
- Replaced `request: gr.Request = None` with the Gradio-recommended
OAuth injection: `token: gr.OAuthToken | None = None`.
- Gradio auto-injects OAuthToken from the user's session if they have
completed the Sign-in-with-HF flow, so we no longer need to manually
read request.session['oauth_info']['access_token'].
- Fallback chain: explicit oauth_token param → injected gr.OAuthToken
→ HF_TOKEN env var.

ALSO FIXED: HF OAuth secret collision
- Removed `hf_oauth: true` and `hf_oauth_scopes` from README frontmatter.
- When hf_oauth: true is set, HF auto-injects OAUTH_CLIENT_ID/
OAUTH_CLIENT_SECRET/OAUTH_SCOPES/OPENID_PROVIDER_URL as env vars,
which collided with the same-named Secrets we set on the Space
for our user-provided OAuth app.
- All 4 OAuth env vars are now provided ONLY as Secrets.

ALSO FIXED: requirements.txt
- Changed `gradio==6.14.0` → `gradio[oauth]==6.14.0` to pull in
authlib + itsdangerous, required by gradio.oauth.attach_oauth().

code/server/routes.py

@@ -34,7 +34,7 @@ import logging
 import os
 import tempfile
 from pathlib import Path
-from typing import Any
+from typing import Any, Optional
 
 import gradio as gr
 from fastapi.responses import HTMLResponse, FileResponse
@@ -541,7 +541,7 @@ def handle_chat(
 @app.api(name="hf_auth", concurrency_limit=4)
 def handle_hf_auth(
     oauth_token: str = "",
-    token: "gr.OAuthToken | None" = None,
+    token: Optional[gr.OAuthToken] = None,
 ) -> str:
     """Get HuggingFace OAuth profile and list of organizations.