feat(subscriptions): dry-run backfill preview — B4 preview (plan #708)

Owning the walk lets an operator gauge "is this source worth a backfill?" before
arming one. ingest_core.Ingester.preview walks the first few feed pages and
counts media NOT already in the seen/dead ledgers, downloading nothing
(read-only). download_backends.preview_source resolves the campaign id + runs it
(native-only, mirrors verify_source_credential / run_download); POST
/api/sources/{id}/preview returns {total_new, posts_scanned, has_more, sample[]}
(409 on auth/drift/unresolvable, 400 for gallery-dl platforms). PatreonClient
gains post_meta(post) for the sample's title/date.

UI: a Patreon-only Preview button (mdi-eye-outline) on SourceRow + SourceCard
opens PreviewDialog — self-fetches with loading / error / empty / result states
and a "Start backfill" shortcut. Store action previewSource.

Tests: preview counts new media without downloading + samples only posts with
new items; page_limit caps the walk + flags has_more.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/subscriptions/PreviewDialog.vue

@@ -0,0 +1,113 @@
+<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">
+      <v-card-title class="d-flex align-center">
+        <v-icon class="mr-2">mdi-eye-outline</v-icon>
+        Preview · {{ source.artist_name }}
+      </v-card-title>
+      <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'
+
+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/components/subscriptions/SourceCard.vue

@@ -61,6 +61,16 @@
             : 'Backfill full history' }}
         </v-tooltip>
       </v-btn>
+      <v-btn
+        v-if="source.platform === 'patreon' && source.backfill_state !== 'running'"
+        size="x-small" variant="text"
+        @click.stop="$emit('preview', source)"
+      >
+        <v-icon>mdi-eye-outline</v-icon>
+        <v-tooltip activator="parent" location="top">
+          Preview — count what a backfill would download (no download)
+        </v-tooltip>
+      </v-btn>
       <v-btn
         v-if="source.platform === 'patreon' && source.backfill_state !== 'running'"
         size="x-small" variant="text"
@@ -98,7 +108,7 @@ const props = defineProps({
   checking: { type: Boolean, default: false },
   warningThreshold: { type: Number, default: 5 },
 })
-const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill', 'recover'])
+const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill', 'recover', 'preview'])
 
 function onToggleEnabled(value) {
   emit('toggle', { source: props.source, enabled: value })

frontend/src/components/subscriptions/SourceRow.vue

@@ -69,6 +69,16 @@
             : 'Backfill — walk the full post history until complete' }}
         </v-tooltip>
       </v-btn>
+      <v-btn
+        v-if="source.platform === 'patreon' && source.backfill_state !== 'running'"
+        size="x-small" variant="text"
+        @click.stop="$emit('preview', source)"
+      >
+        <v-icon>mdi-eye-outline</v-icon>
+        <v-tooltip activator="parent" location="top">
+          Preview — count what a backfill would download (no download)
+        </v-tooltip>
+      </v-btn>
       <v-btn
         v-if="source.platform === 'patreon' && source.backfill_state !== 'running'"
         size="x-small" variant="text"
@@ -106,7 +116,7 @@ const props = defineProps({
   checking: { type: Boolean, default: false },
   warningThreshold: { type: Number, default: 5 },
 })
-const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill', 'recover'])
+const emit = defineEmits(['edit', 'remove', 'toggle', 'check', 'backfill', 'recover', 'preview'])
 
 function onToggleEnabled(value) {
   emit('toggle', { source: props.source, enabled: value })

frontend/src/components/subscriptions/SubscriptionsTab.vue

@@ -176,6 +176,7 @@
                     @check="onCheck"
                     @backfill="onBackfill"
                     @recover="onRecover"
+                    @preview="onPreview"
                   />
                   <tr v-if="item.sources.length === 0">
                     <td colspan="8" class="fc-subs__sources-empty">
@@ -253,6 +254,7 @@
             @check="onCheck"
             @backfill="onBackfill"
             @recover="onRecover"
+            @preview="onPreview"
           />
           <div v-if="item.sources.length === 0" class="fc-subs__sources-empty">
             No sources yet. Tap + to add one.
@@ -268,6 +270,11 @@
       @saved="onSourceSaved"
     />
     <ArtistCreateDialog v-model="showArtistDialog" @created="onArtistCreated" />
+    <PreviewDialog
+      v-model="showPreviewDialog"
+      :source="previewTarget"
+      @backfill="onPreviewBackfill"
+    />
   </div>
 </template>
 
@@ -284,6 +291,7 @@ import SourceCard from './SourceCard.vue'
 import SourceHealthDot from './SourceHealthDot.vue'
 import SourceFormDialog from './SourceFormDialog.vue'
 import ArtistCreateDialog from './ArtistCreateDialog.vue'
+import PreviewDialog from './PreviewDialog.vue'
 import PlatformChip from './PlatformChip.vue'
 import SchedulerStatusBar from './SchedulerStatusBar.vue'
 import { formatRelative } from '../../utils/date.js'
@@ -334,6 +342,8 @@ const showSourceDialog = ref(false)
 const editingSource = ref(null)
 const editingArtist = ref(null)
 const showArtistDialog = ref(false)
+const showPreviewDialog = ref(false)
+const previewTarget = ref(null)
 
 const artistFilter = computed(() => {
   const raw = route.query.artist_id
@@ -572,6 +582,18 @@ async function onRecover(source) {
   }
 }
 
+// Plan #708 B4: open the dry-run preview dialog (it self-fetches on open).
+function onPreview(source) {
+  previewTarget.value = source
+  showPreviewDialog.value = true
+}
+
+// "Start backfill" from inside the preview dialog → arm it + close.
+async function onPreviewBackfill(source) {
+  showPreviewDialog.value = false
+  await onBackfill(source)
+}
+
 async function checkAll(group) {
   let ok = 0
   let conflict = 0

frontend/src/stores/sources.js

@@ -107,6 +107,12 @@ 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) ?? []
@@ -136,6 +142,7 @@ export const useSourcesStore = defineStore('sources', () => {
     startBackfill,
     stopBackfill,
     recoverSource,
+    previewSource,
     findOrCreateArtist, autocompleteArtist,
     loadScheduleStatus,
     sourcesByArtistGrouped,