feat(tags): retro-normalize existing tags to Title Case + merge case-collisions (plan #714)

Follow-up to #701: new tags are saved canonical, but the back-catalog keeps
whatever casing it was created with. This adds a maintenance action that
Title-Cases every existing tag (collapsing whitespace) and merges
case/whitespace-variant duplicates into one.

Backend:
- tag_service.normalize_existing_tags(session, *, dry_run): groups all tags by
  (kind, coalesce(fandom_id,-1), canonical_name). Per group it picks a survivor
  (prefer an already-canonical member → no rename/self-alias; else the
  best-connected tag → fewest FK repoints; else lowest id), merges the variants
  INTO it via the tested TagService._do_merge (image_tag/allowlist/embedding/
  aliases/series_page repoints + protective ML aliases), then renames the
  survivor to canonical. Losers are deleted before the rename so there's no
  transient unique-index clash; commits per group and isolates failures per
  group. Idempotent — an already-canonical lone tag is a no-op.
- normalize_tags_task (maintenance queue, asyncio.run + per-task NullPool async
  engine, soft 1800/hard 2400) — recovery/timeout/duration covered by FC-3i.
- POST /api/admin/tags/normalize: dry_run=true returns a projection inline
  (group/collision/rename counts + sample); dry_run=false enqueues the task.

Frontend: a "Standardize tag casing" section in TagMaintenanceCard (Cleanup
tab) — preview → apply (polls the activity dashboard to terminal status),
behind a back-up-first warning. admin store gains normalizeTags().

Tests: tests/test_tag_normalize.py — dry-run counts, live merge + image-tag
dedup/repoint, idempotency, same-name-different-fandom and -different-kind kept
separate, ML-known loser keeps a protective alias.

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

frontend/src/components/settings/TagMaintenanceCard.vue

@@ -136,6 +136,64 @@
         >Delete {{ resetPreview.count }} content tag(s) +
           {{ resetPreview.applications }} application(s)</v-btn>
       </div>
+
+      <v-divider class="my-5" />
+
+      <!-- #714: standardize existing tags to the canonical Title-Case form
+           and merge case/whitespace-variant duplicates. -->
+      <p class="text-body-2 mb-2">
+        <strong>Standardize tag casing.</strong>
+        New tags are saved Title Case, but older tags keep whatever casing they
+        were created with. This renames every existing tag to
+        <code>Title Case</code> (collapsing extra spaces) and
+        <strong>merges</strong> tags that differ only by case or spacing
+        (e.g. <code>hatsune miku</code> + <code>Hatsune&nbsp;&nbsp;Miku</code>)
+        into one — repointing their images, allowlist entries and series pages.
+      </p>
+      <v-alert type="warning" variant="tonal" density="compact" class="mb-3">
+        The merges are irreversible — back up first
+        (Settings → Maintenance → Backup). Safe to run more than once.
+      </v-alert>
+
+      <v-btn
+        color="accent" variant="flat" rounded="pill"
+        prepend-icon="mdi-magnify"
+        :loading="loadingNormPreview"
+        class="mb-3"
+        @click="onNormPreview"
+      >Preview tag standardization</v-btn>
+
+      <div v-if="normPreview">
+        <p class="text-body-2 mb-2">
+          <strong>{{ normPreview.total_changes }}</strong> tag group(s) to
+          change — <strong>{{ normPreview.tags_to_rename }}</strong> rename(s),
+          <strong>{{ normPreview.collisions }}</strong> collision(s) merging
+          <strong>{{ normPreview.tags_to_merge }}</strong> tag(s) away.
+        </p>
+        <div v-if="normPreview.sample?.length" class="fc-name-grid mb-3">
+          <span
+            v-for="s in normPreview.sample" :key="s.to + s.kind"
+            class="fc-name"
+          >
+            <template v-if="s.merge">{{ s.from.join(' + ') }} → </template>{{ s.to }}
+          </span>
+        </div>
+        <v-btn
+          color="error" variant="flat" rounded="pill"
+          prepend-icon="mdi-format-letter-case"
+          :disabled="!normPreview.total_changes"
+          :loading="normCommitting"
+          @click="onNormCommit"
+        >Standardize {{ normPreview.total_changes }} tag group(s)</v-btn>
+        <span
+          v-if="normResult"
+          class="ml-3 text-caption"
+          :class="normResult === 'ok' ? 'text-success' : 'text-warning'"
+        >
+          <template v-if="normResult === 'ok'">Standardization complete ✓</template>
+          <template v-else>Finished with status: {{ normResult }}</template>
+        </span>
+      </div>
     </v-card-text>
   </v-card>
 </template>
@@ -155,6 +213,10 @@ const kindCommitting = ref(false)
 const resetPreview = ref(null)
 const loadingResetPreview = ref(false)
 const resetCommitting = ref(false)
+const normPreview = ref(null)
+const loadingNormPreview = ref(false)
+const normCommitting = ref(false)
+const normResult = ref(null)
 
 async function onPreview() {
   loadingPreview.value = true
@@ -212,6 +274,33 @@ async function onResetCommit() {
     resetCommitting.value = false
   }
 }
+
+async function onNormPreview() {
+  loadingNormPreview.value = true
+  normResult.value = null
+  try {
+    normPreview.value = await store.normalizeTags({ dryRun: true })
+  } finally {
+    loadingNormPreview.value = false
+  }
+}
+
+async function onNormCommit() {
+  normCommitting.value = true
+  normResult.value = null
+  try {
+    // Long op (FK repoints): enqueue the maintenance task, then tail the
+    // activity dashboard until its row reaches a terminal status. (The
+    // per-run summary dict isn't exposed by /activity/runs, so we surface
+    // the terminal status — the dry-run preview is the detailed view.)
+    const { task_id: taskId } = await store.normalizeTags({ dryRun: false })
+    const row = await store.pollTaskUntilDone(taskId)
+    normResult.value = row?.status || 'ok'
+    normPreview.value = { total_changes: 0, tags_to_rename: 0, collisions: 0, tags_to_merge: 0, sample: [] }
+  } finally {
+    normCommitting.value = false
+  }
+}
 </script>
 
 <style scoped>

frontend/src/stores/admin.js

@@ -142,6 +142,22 @@ export const useAdminStore = defineStore('admin', () => {
     }
   }
 
+  // #714: Title-Case the back-catalog + merge case/whitespace-variant tags.
+  // dry-run returns a projection inline; live returns {task_id} (long op —
+  // FK repoints) the caller polls via pollTaskUntilDone.
+  async function normalizeTags({ dryRun = true } = {}) {
+    lastError.value = null
+    try {
+      return await api.post(
+        '/api/admin/tags/normalize',
+        { body: { dry_run: dryRun } },
+      )
+    } catch (e) {
+      lastError.value = e.message
+      throw e
+    }
+  }
+
   // --- Task progress polling (taps FC-3i activity dashboard) --------
 
   /**
@@ -178,6 +194,7 @@ export const useAdminStore = defineStore('admin', () => {
     pruneUnusedTags,
     purgeLegacyTags,
     resetContentTagging,
+    normalizeTags,
     pollTaskUntilDone,
   }
 })