add schema version endpoint

schema.graphqls

@@ -87,6 +87,8 @@ type Query {
 
   isNotificationScheduled(input: NotificationInput!): Boolean
   secondsThresholdForNotification(input: NotificationInput!): Int
+
+  schemaVersion: ID!
 }
 
 # Mutations

src/resolvers/QueryResolvers.ts

@@ -1,6 +1,8 @@
 import { ServerContext } from "../ServerContext";
 import { Resolvers } from "../generated/graphql";
 
+const GRAPHQL_SCHEMA_VERSION = "1";
+
 export const QueryResolvers: Resolvers<ServerContext> = {
   Query: {
     systems: async (_parent, args, contextValue, _info) => {
@@ -46,5 +48,8 @@ export const QueryResolvers: Resolvers<ServerContext> = {
 
       return null;
     },
+    schemaVersion: async (_parent, _args, _contextValue, _info) => {
+      return GRAPHQL_SCHEMA_VERSION;
+    },
   },
 }