docs(adr-015): cross-feature events and background jobs

Also folds in the spec correction noted in the plan's known follow-up
\#1: subscribe takes consumerFeature: string as a second arg between
descriptor and handler. § 3.3 and § 5.4 updated.

docs/superpowers/specs/2026-05-08-events-and-jobs-design.md

@@ -141,6 +141,7 @@ export interface IEventBus {
 
   subscribe<T>(
     descriptor: EventDescriptor<string, z.ZodType<T>>,
+    consumerFeature: string,
     handler: EventHandler<T>,
   ): void;
 }
@@ -321,7 +322,7 @@ export function bindProductionMarketingPages(config: SanitizedConfig): void {
       onAuthUserSignedUpHandler(queue),
     ),
   );
-  bus.subscribe(userSignedUpEvent, onAuthUserSignedUp);
+  bus.subscribe(userSignedUpEvent, "marketing-pages", onAuthUserSignedUp);
 }
 ```