server: system accounts cannot be registered

While refactoring the previous commit, it seemed like the previous
authors expected that a system account could be registered somehow
and that this would be an error condition. However, as now made
explicit with this, it is not possible to register a system account.

This means that any account by that name could only ever have been
created by the system itself so fetching them should be fine and not
an error condition.

packages/backend/src/models/repositories/user.ts

@@ -26,6 +26,8 @@ type IsMeAndIsUserDetailed<ExpectsMe extends boolean | null, Detailed extends bo
 
 const ajv = new Ajv();
 
+// It is important that localUsernameSchema does not allow any usernames
+// containing dots because those are used for system actors.
 const localUsernameSchema = { type: 'string', pattern: /^\w{1,20}$/.toString().slice(1, -1) } as const;
 const passwordSchema = { type: 'string', minLength: 1 } as const;
 const nameSchema = { type: 'string', minLength: 1, maxLength: 50 } as const;