add password-based login endpoint

Foxchat.Identity/Middleware/AuthenticationMiddleware.cs

@@ -71,6 +71,13 @@ public static class HttpContextExtensions
             return token as Token;
         return null;
     }
+
+    public static Application GetApplicationOrThrow(this HttpContext context)
+    {
+        var token = context.GetToken();
+        if (token is not { Account: null }) throw new ApiError.Forbidden("This endpoint requires a client token.");
+        return token.Application;
+    }
 }
 
 [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]