adjust steam auth, make neutrino post only

src/API/Controllers/Neutrino/NeutrinoController.cs

@@ -18,7 +18,7 @@ public class NeutrinoController(
     private readonly NeutrinoOptions _options = options.Value;
     
     // Photon sends the request with Content-Type: text/plain instead of application/json.
-    [Route("authorize")]
+    [HttpPost("authorize")]
     public async Task<ActionResult<NeutrinoAuthenticateResponse>> AuthorizeNeutrinoAsync(
         [FromQuery(Name = "secret")] string? secret = null,
         CancellationToken ct = default)