refactor(subscriptions): remove the dead backfill dry-run preview (#1281)

PreviewDialog.vue was orphaned — nothing mounted it — so the dry-run preview
was unreachable. Rather than wire it up, remove the whole chain: its unique
value (a capped 3-page count of what a backfill would grab) is low, and its
adjacent needs are already covered — auth validation by verify_source_credential,
and actually fetching recent posts by "Check now". Operator decision 2026-07-06.

Removed:
- frontend: PreviewDialog.vue + sources store previewSource()
- backend: POST /api/sources/<id>/preview route, download_backends.preview_source,
  IngestCore.preview() + its now-unused NativeIngestError import
- tests: the 3 ingester preview tests

Nothing else referenced the chain (verified). Shared campaign-resolution and
ledger helpers stay — they're used by run()/verify_source_credential.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/components/subscriptions/PreviewDialog.vue

@@ -1,114 +0,0 @@
-<template>
-  <!-- Plan #708 B4: dry-run preview — what a backfill WOULD download, without
-       downloading. Self-fetches on open; loading / error / empty / result. -->
-  <v-dialog
-    :model-value="modelValue" max-width="520"
-    @update:model-value="$emit('update:modelValue', $event)"
-  >
-    <v-card v-if="source">
-      <CardHeading
-        icon="mdi-eye-outline"
-        :title="`Preview · ${source.artist_name}`"
-      />
-      <v-card-subtitle class="fc-preview__url">{{ source.url }}</v-card-subtitle>
-
-      <v-card-text>
-        <div v-if="loading" class="text-center py-8">
-          <v-progress-circular indeterminate color="accent" />
-          <div class="text-caption mt-3 text-medium-emphasis">Walking the feed…</div>
-        </div>
-
-        <v-alert
-          v-else-if="error" type="error" variant="tonal" density="comfortable"
-        >{{ error }}</v-alert>
-
-        <div v-else-if="result">
-          <div class="text-h5">
-            {{ result.total_new }} new item{{ result.total_new === 1 ? '' : 's' }}
-          </div>
-          <div class="text-body-2 text-medium-emphasis mb-3">
-            in the {{ result.posts_scanned }} most recent
-            post{{ result.posts_scanned === 1 ? '' : 's' }}<span
-              v-if="result.has_more"> · more pages not scanned</span>
-          </div>
-
-          <div
-            v-if="result.total_new === 0"
-            class="text-body-2 text-medium-emphasis"
-          >You’re caught up — nothing new to download.</div>
-
-          <v-list v-else density="compact" class="fc-preview__list" bg-color="transparent">
-            <v-list-item v-for="(row, i) in result.sample" :key="i" class="px-0">
-              <template #prepend>
-                <v-chip
-                  size="x-small" color="info" variant="tonal" label class="mr-3"
-                >+{{ row.new }}</v-chip>
-              </template>
-              <v-list-item-title class="text-body-2">{{ row.title }}</v-list-item-title>
-              <v-list-item-subtitle v-if="row.date" class="text-caption">
-                {{ formatRelative(row.date) }}
-              </v-list-item-subtitle>
-            </v-list-item>
-          </v-list>
-        </div>
-      </v-card-text>
-
-      <v-card-actions>
-        <v-spacer />
-        <v-btn variant="text" @click="$emit('update:modelValue', false)">Close</v-btn>
-        <v-btn
-          v-if="result && result.total_new > 0"
-          color="accent" variant="flat"
-          @click="$emit('backfill', source)"
-        >Start backfill</v-btn>
-      </v-card-actions>
-    </v-card>
-  </v-dialog>
-</template>
-
-<script setup>
-import { ref, watch } from 'vue'
-import { useSourcesStore } from '../../stores/sources.js'
-import { formatRelative } from '../../utils/date.js'
-import CardHeading from '../common/CardHeading.vue'
-
-const props = defineProps({
-  modelValue: { type: Boolean, default: false },
-  source: { type: Object, default: null },
-})
-defineEmits(['update:modelValue', 'backfill'])
-
-const store = useSourcesStore()
-const loading = ref(false)
-const error = ref('')
-const result = ref(null)
-
-watch(
-  () => props.modelValue,
-  async (open) => {
-    if (!open || !props.source) return
-    loading.value = true
-    error.value = ''
-    result.value = null
-    try {
-      result.value = await store.previewSource(props.source.id)
-    } catch (e) {
-      error.value = e?.body?.detail || e?.message || 'Preview failed'
-    } finally {
-      loading.value = false
-    }
-  },
-)
-</script>
-
-<style scoped>
-.fc-preview__url {
-  overflow: hidden;
-  text-overflow: ellipsis;
-  white-space: nowrap;
-}
-.fc-preview__list {
-  max-height: 320px;
-  overflow-y: auto;
-}
-</style>

frontend/src/stores/sources.js

@@ -149,12 +149,6 @@ export const useSourcesStore = defineStore('sources', () => {
     return body
   }
 
-  // Plan #708 B4: dry-run preview — count what a backfill WOULD download
-  // (native platforms), without downloading. Read-only, so no cache invalidate.
-  async function previewSource(id) {
-    return await api.post(`/api/sources/${id}/preview`)
-  }
-
   function sourcesByArtistGrouped() {
     // returns [{artist: {id,name,slug}, sources: [...]}, ...]
     const arr = byArtist.value.get(null) ?? []
@@ -185,7 +179,6 @@ export const useSourcesStore = defineStore('sources', () => {
     stopBackfill,
     recoverSource,
     recaptureSource,
-    previewSource,
     findOrCreateArtist, autocompleteArtist, reassign,
     loadScheduleStatus,
     sourcesByArtistGrouped,