fix: enhance auth_callback to support client authentication via POST body for OIDC providers

backend/app/main.py

@@ -89,6 +89,13 @@ async def auth_callback(payload: AuthCallbackRequest):
     async with httpx.AsyncClient(timeout=15.0) as client:
         response = await client.post(token_endpoint, data=data, headers={"Accept": "application/json"}, auth=auth)
 
+        # Some providers only accept client authentication via POST body
+        if response.status_code != 200 and auth is not None:
+            # try falling back to client_secret in the request body
+            data_with_secret = dict(data)
+            data_with_secret["client_secret"] = settings.oidc_client_secret
+            response = await client.post(token_endpoint, data=data_with_secret, headers={"Accept": "application/json"})
+
     if response.status_code != 200:
         raise HTTPException(status_code=400, detail=f"OIDC token exchange failed: {response.text}")