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/components/subscriptions/PosterCleanupDialog.vue

@@ -0,0 +1,135 @@
+<template>
+  <!-- #4486. A Discord source's posts by people outside its "Only posts by"
+       list, taken before the list existed. Preview first, always: nothing is
+       deleted until the operator has seen the per-poster breakdown and typed
+       the token that preview returned. -->
+  <v-dialog :model-value="modelValue" max-width="560"
+            @update:model-value="$emit('update:modelValue', $event)">
+    <v-card>
+      <v-card-title>Remove posts from other posters</v-card-title>
+      <v-card-text>
+        <div class="text-caption mb-2">{{ source.display_name || source.url }}</div>
+        <v-progress-linear v-if="loading" indeterminate color="accent" class="mb-3" />
+        <v-alert v-else-if="error" type="warning" variant="tonal" density="compact">
+          {{ error }}
+        </v-alert>
+        <template v-else-if="result">
+          <v-alert type="success" variant="tonal" density="compact">
+            Removed {{ result.posts_deleted }} post(s), {{ result.images_deleted }} image(s),
+            {{ result.attachments_deleted }} attachment(s) and {{ result.drops_deleted }}
+            grouped post(s). Grouping rebuilds what remains on its next sweep.
+          </v-alert>
+        </template>
+        <template v-else-if="preview">
+          <p class="mb-2">
+            Keeping posts by <strong>{{ preview.authors.join(', ') }}</strong>.
+          </p>
+          <p v-if="!preview.posts" class="fc-dim">
+            Nothing to remove: every post with a known poster is by someone on the list.
+          </p>
+          <template v-else>
+            <v-table density="compact" class="mb-3">
+              <thead><tr><th>Poster</th><th class="text-right">Posts</th></tr></thead>
+              <tbody>
+                <tr v-for="p in preview.posters" :key="p.poster">
+                  <td>{{ p.poster }}</td>
+                  <td class="text-right">{{ p.posts }}</td>
+                </tr>
+              </tbody>
+            </v-table>
+            <p class="mb-1">
+              {{ preview.posts }} post(s), {{ preview.images }} image(s) found only on them,
+              {{ preview.attachments }} attachment(s).
+            </p>
+            <p v-if="preview.drops" class="mb-1">
+              {{ preview.drops }} grouped post(s) that include them are ungrouped; the
+              rest regroup on the next sweep.
+            </p>
+            <p class="fc-dim">
+              Images that also belong to a kept post stay. If a name you want to keep
+              appears above, add it to the source's "Only posts by" list first.
+            </p>
+          </template>
+          <p v-if="preview.unknown_posts" class="fc-dim mt-2">
+            {{ preview.unknown_posts }} older post(s) don't record who posted them and
+            are left alone.
+          </p>
+        </template>
+      </v-card-text>
+      <v-card-actions>
+        <v-spacer />
+        <v-btn variant="text" @click="$emit('update:modelValue', false)">
+          {{ result ? 'Close' : 'Cancel' }}
+        </v-btn>
+        <v-btn
+          v-if="preview && preview.posts && !result"
+          color="error" variant="flat" :loading="busy"
+          @click="confirmOpen = true"
+        >Remove {{ preview.posts }} post(s)</v-btn>
+      </v-card-actions>
+    </v-card>
+    <DestructiveConfirmModal
+      v-if="preview"
+      v-model="confirmOpen"
+      action="delete" kind="other posters' posts" tier="C"
+      :expected-token-override="preview.confirm_token"
+      :projected-counts="{ posts: preview.posts, images: preview.images,
+                           attachments: preview.attachments, grouped: preview.drops }"
+      @confirm="onConfirm"
+    />
+  </v-dialog>
+</template>
+
+<script setup>
+import { ref, watch } from 'vue'
+import { useSourcesStore } from '../../stores/sources.js'
+import DestructiveConfirmModal from '../modal/DestructiveConfirmModal.vue'
+
+const props = defineProps({
+  modelValue: { type: Boolean, default: false },
+  source: { type: Object, required: true },
+})
+defineEmits(['update:modelValue'])
+const store = useSourcesStore()
+
+const loading = ref(false)
+const busy = ref(false)
+const error = ref('')
+const preview = ref(null)
+const result = ref(null)
+const confirmOpen = ref(false)
+
+function message(e) {
+  return e?.body?.detail || e?.body?.error || e?.message || String(e)
+}
+
+watch(() => props.modelValue, async (open) => {
+  if (!open) return
+  preview.value = null
+  result.value = null
+  error.value = ''
+  loading.value = true
+  try {
+    preview.value = await store.previewOtherPosters(props.source.id)
+  } catch (e) {
+    error.value = message(e)
+  } finally {
+    loading.value = false
+  }
+})
+
+async function onConfirm(token) {
+  busy.value = true
+  try {
+    result.value = await store.removeOtherPosters(props.source.id, token)
+  } catch (e) {
+    error.value = message(e)
+  } finally {
+    busy.value = false
+  }
+}
+</script>
+
+<style scoped>
+.fc-dim { color: rgb(var(--v-theme-on-surface-variant)); }
+</style>