chore: replace systemUser -1 with new -1337 system user (#5999)

## About the changes
This PR replaces the old systemUser -1 in user-service.ts with the new
SYSTEM_USER -1337 and adds a migration to move events created_by = -1 to
-1337


## Discussion points
Does it make sense to do both of these things? Or should we skip the
migration? How would this behave in a large system with hundreds of
thousands of events, should this be split up?

src/lib/services/user-service.ts

@@ -33,7 +33,7 @@ import { TokenUserSchema } from '../openapi/spec/token-user-schema';
 import PasswordMismatch from '../error/password-mismatch';
 import EventService from '../features/events/event-service';
 
-const systemUser = new User({ id: -1, username: 'system' });
+import { SYSTEM_USER } from '../types';
 
 export interface ICreateUser {
     name?: string;
@@ -244,7 +244,7 @@ class UserService {
         return userCreated;
     }
 
-    private getCreatedBy(updatedBy: IUser = systemUser) {
+    private getCreatedBy(updatedBy: IUser = new User(SYSTEM_USER)): string {
         return updatedBy.username || updatedBy.email;
     }
 
@@ -444,7 +444,10 @@ class UserService {
 
     async createResetPasswordEmail(
         receiverEmail: string,
-        user: IUser = systemUser,
+        user: IUser = new User({
+            id: SYSTEM_USER.id,
+            username: SYSTEM_USER.username,
+        }),
     ): Promise<URL> {
         const receiver = await this.getByEmail(receiverEmail);
         if (!receiver) {

src/migrations/20240125100000-events-system-user-old2new.js

@@ -0,0 +1,14 @@
+'use strict';
+
+exports.up = function (db, callback) {
+    db.runSql(
+        `
+        UPDATE events SET created_by_user_id = -1337 WHERE created_by_user_id = -1;
+        `,
+        callback,
+    );
+};
+
+exports.down = function (db, callback) {
+    callback();
+};