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>

frontend/src/stores/sources.js

@@ -149,6 +149,15 @@ export const useSourcesStore = defineStore('sources', () => {
     return body
   }
 
+  // #4486: a Discord source's posts by people outside its poster list.
+  // The preview touches nothing and hands back the token the remove must echo.
+  async function previewOtherPosters(id) {
+    return api.get(`/api/sources/${id}/discord/other-posters`)
+  }
+  async function removeOtherPosters(id, confirm) {
+    return api.post(`/api/sources/${id}/discord/other-posters/remove`, { body: { confirm } })
+  }
+
   function sourcesByArtistGrouped() {
     // returns [{artist: {id,name,slug}, sources: [...]}, ...]
     const arr = byArtist.value.get(null) ?? []
@@ -179,6 +188,7 @@ export const useSourcesStore = defineStore('sources', () => {
     stopBackfill,
     recoverSource,
     recaptureSource,
+    previewOtherPosters, removeOtherPosters,
     findOrCreateArtist, autocompleteArtist, reassign,
     loadScheduleStatus,
     sourcesByArtistGrouped,