feat: a Discord source can remove the posts it took from people outside its poster list (#4486)

"Remove posts from other posters" on a Discord source with an "Only posts by"
list previews what goes, per poster, then deletes it behind a typed token:

- posts whose record names a poster not on the list (by id, username or
  display name); posts that record no poster are left alone and counted;
- images found only on those posts; one also on a kept post stays, and a
  synthetic drop's link never keeps one;
- their attachments, and every drop that absorbed one of them, so the grouping
  sweep regroups what remains.

Preview and apply share one predicate, and a parity test holds them to it.
The post record now also saves the poster's display name.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/services/discord_downloader.py

@@ -195,6 +195,7 @@ class DiscordDownloader(BaseNativeDownloader):
             "parent": meta.get("parent"),
             "is_thread": meta.get("is_thread"),
             "author": author.get("username"),
+            "author_name": author.get("global_name"),
             "author_id": author.get("id"),
             "message": body,
             "date": post.get("timestamp"),