fix: exclude soft-deleted posts from outbox and content negotiation

Deleted posts (with properties.deleted timestamp) were still served
via the outbox dispatcher and content negotiation catch-all. Now:
- Outbox find() and countDocuments() filter out deleted posts
- Object dispatcher returns null for deleted posts (Fedify 404)
- Content negotiation falls through to Express for deleted posts

Confab-Link: http://localhost:8080/sessions/af5f8b45-6b8d-442d-8f25-78c326190709

index.js

@@ -434,7 +434,7 @@ export default class ActivityPubEndpoint {
           "properties.url": requestUrl,
         });
 
-        if (!post) {
+        if (!post || post.properties?.deleted) {
           return next();
         }