feat(maintenance): reconcile duplicate posts (gallery-dl→native unify)

An artist first downloaded by gallery-dl gets Post rows keyed by the per-
attachment id; a later native walk keys the SAME real post by the post id. They
never dedup (uq_post_source_external_id is on external_post_id) → duplicate post
rows (cheunart: 943→1109). The real post id is recoverable in-DB from
raw_metadata['post_id'] (both eras store the sidecar there).

reconcile_duplicate_posts (cleanup_service): group posts by (source_id, canonical
post_id = raw_metadata.post_id else external_post_id); for each group >1, keep the
row already keyed by the post id (the format the CURRENT native downloader
produces, so future walks dedup and this can't recur), re-point
ImageRecord.primary_post_id / ImageProvenance / PostAttachment / ExternalLink onto
it conflict-safe (drop the loser's row where the keeper already has the equivalent,
per each table's uniqueness), backfill the keeper's empty date/title/body/raw_meta
from a loser, set external_post_id=post_id + derive post_url, delete losers.
IMAGES ARE NOT TOUCHED (content-addressed/deduped already; operator-confirmed).

Preview/apply share find_duplicate_post_groups (rule 93). API
/api/admin/posts/reconcile-duplicates (dry_run→{groups,posts_to_merge,sample};
apply→{groups,merged,sample}; optional source_id). UI: a second section on
PostMaintenanceCard (preview groups+sample → confirm merge). Tests: merge +
metadata backfill + image move, no-op when unique, provenance-collision dedup.

Design: milestone #73. Forensics: note #917. Out of scope (flagged): cheunart vs
Cheunart case-variant artist dirs/rows.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/settings/PostMaintenanceCard.vue

@@ -49,12 +49,56 @@
           class="ml-3 text-caption text-success"
         >Deleted {{ deleted }} ✓</span>
       </div>
+
+      <v-divider class="my-5" />
+
+      <p class="fc-muted text-body-2 mb-4">
+        Unify <strong>duplicate posts</strong> — when an artist was first
+        downloaded by gallery-dl and later re-walked by the native ingester, the
+        same post can exist twice (once keyed by the file's attachment id, once by
+        the real post id). This merges each pair onto one canonical post (keeping
+        the native post-id key), moving images, attachments and links onto the
+        survivor. Images themselves are never touched.
+      </p>
+
+      <v-btn
+        color="accent" variant="flat" rounded="pill"
+        prepend-icon="mdi-magnify"
+        :loading="loadingDupPreview"
+        class="mb-3"
+        @click="onPreviewDupes"
+      >Preview duplicate posts</v-btn>
+
+      <div v-if="dupPreview">
+        <p class="text-body-2 mb-2">
+          <strong>{{ dupPreview.groups }}</strong> duplicate group(s),
+          <strong>{{ dupPreview.posts_to_merge }}</strong> redundant row(s) to
+          merge.
+          <span v-if="dupPreview.groups > 50" class="fc-muted">Showing first 50.</span>
+          <span v-else-if="!dupPreview.groups" class="fc-muted">No duplicates found.</span>
+        </p>
+        <SampleNameGrid
+          v-if="dupSampleNames.length"
+          :names="dupSampleNames" class="mb-3"
+        />
+        <v-btn
+          color="error" variant="flat" rounded="pill"
+          prepend-icon="mdi-merge"
+          :disabled="!dupPreview.groups"
+          :loading="merging"
+          @click="onMergeDupes"
+        >Merge {{ dupPreview.posts_to_merge }} duplicate(s)</v-btn>
+        <span
+          v-if="merged != null"
+          class="ml-3 text-caption text-success"
+        >Merged {{ merged }} ✓</span>
+      </div>
     </v-card-text>
   </v-card>
 </template>
 
 <script setup>
-import { ref } from 'vue'
+import { computed, ref } from 'vue'
 
 import { useAdminStore } from '../../stores/admin.js'
 import SampleNameGrid from '../common/SampleNameGrid.vue'
@@ -66,6 +110,16 @@ const loadingPreview = ref(false)
 const committing = ref(false)
 const deleted = ref(null)
 
+const dupPreview = ref(null)
+const loadingDupPreview = ref(false)
+const merging = ref(false)
+const merged = ref(null)
+const dupSampleNames = computed(() =>
+  (dupPreview.value?.sample ?? []).map(
+    (g) => `${g.title} — ${g.rows} rows`,
+  ),
+)
+
 async function onPreview() {
   loadingPreview.value = true
   deleted.value = null
@@ -88,6 +142,28 @@ async function onCommit() {
     committing.value = false
   }
 }
+
+async function onPreviewDupes() {
+  loadingDupPreview.value = true
+  merged.value = null
+  try {
+    dupPreview.value = await store.reconcileDuplicatePosts({ dryRun: true })
+  } finally {
+    loadingDupPreview.value = false
+  }
+}
+
+async function onMergeDupes() {
+  merging.value = true
+  try {
+    const result = await store.reconcileDuplicatePosts({ dryRun: false })
+    merged.value = result.merged ?? 0
+    // Same predicate as the preview — after the merge there are no dup groups left.
+    dupPreview.value = { groups: 0, posts_to_merge: 0, sample: [] }
+  } finally {
+    merging.value = false
+  }
+}
 </script>
 
 <style scoped>

frontend/src/stores/admin.js

@@ -129,6 +129,22 @@ export const useAdminStore = defineStore('admin', () => {
     }
   }
 
+  // Tier-A: unify duplicate post rows (gallery-dl attachment-id + native post-id
+  // for the same real post) onto one post-id-keyed keeper. Images untouched.
+  // Preview/apply parity on the backend; UI previews then confirms.
+  async function reconcileDuplicatePosts({ dryRun = true, sourceId = null } = {}) {
+    lastError.value = null
+    try {
+      return await api.post(
+        '/api/admin/posts/reconcile-duplicates',
+        { body: { dry_run: dryRun, source_id: sourceId } },
+      )
+    } catch (e) {
+      lastError.value = e.message
+      throw e
+    }
+  }
+
   async function purgeLegacyTags({ dryRun = true } = {}) {
     lastError.value = null
     try {
@@ -208,6 +224,7 @@ export const useAdminStore = defineStore('admin', () => {
     tagUsageCount,
     pruneUnusedTags,
     pruneBarePosts,
+    reconcileDuplicatePosts,
     purgeLegacyTags,
     resetContentTagging,
     normalizeTags,