feat(tags): 'Reset content tagging' admin action

Wipe every general + character tag so the operator can re-tag from scratch via
the Camie auto-suggest, while PRESERVING fandoms, series (+ series_page order),
and each image's stored tagger_predictions (so suggestions repopulate
immediately). One set-based DELETE FROM tag WHERE kind IN ('general','character')
— the five tag-referencing tables all cascade, so applications + aliases +
allowlist + rejections + centroids clear automatically; series tags aren't
deleted so series survive; Tag.fandom_id is SET NULL so fandoms are untouched.

Reuses the established dry-run-preview -> confirm pattern: cleanup_service.
reset_content_tagging() + POST /api/admin/tags/reset-content +
TagMaintenanceCard section with a backup-first warning and a red confirm
showing exact counts (tags by kind + image applications). Irreversible except
via DB backup restore; the wipe only fires when the operator confirms.

Tests: service dry-run counts + live delete preserves fandom/series/series_page
while content tags + their image_tag cascade away; API dry-run wiring.

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

frontend/src/components/settings/TagMaintenanceCard.vue

@@ -89,6 +89,53 @@
           @click="onKindCommit"
         >Delete {{ kindPreview.count }} legacy tag(s)</v-btn>
       </div>
+
+      <v-divider class="my-5" />
+
+      <p class="text-body-2 mb-2">
+        <strong class="text-error">Reset content tagging.</strong>
+        Deletes every <code>general</code> and <code>character</code> tag and
+        removes them from every image, so you can re-tag from scratch with the
+        auto-suggest. <strong>Fandoms and series (with their page order) are
+        kept</strong>, and each image's saved predictions are untouched — open
+        an image and its suggestions reappear.
+      </p>
+      <v-alert type="warning" variant="tonal" density="compact" class="mb-3">
+        Irreversible — there's no undo except restoring a DB backup.
+        Back one up first (Settings → Maintenance → Backup).
+      </v-alert>
+
+      <v-btn
+        color="accent" variant="flat" rounded="pill"
+        prepend-icon="mdi-magnify"
+        :loading="loadingResetPreview"
+        class="mb-3"
+        @click="onResetPreview"
+      >Preview content-tag reset</v-btn>
+
+      <div v-if="resetPreview">
+        <p class="text-body-2 mb-2">
+          <strong>{{ resetPreview.count }}</strong> content tag(s)
+          <span v-for="(n, k) in resetPreview.by_kind" :key="k" class="fc-muted">
+            ({{ k }}: {{ n }})&nbsp;
+          </span>
+          across <strong>{{ resetPreview.applications }}</strong> image
+          application(s).
+        </p>
+        <div v-if="resetPreview.sample_names?.length" class="fc-name-grid mb-3">
+          <span v-for="n in resetPreview.sample_names" :key="n" class="fc-name">
+            {{ n }}
+          </span>
+        </div>
+        <v-btn
+          color="error" variant="flat" rounded="pill"
+          prepend-icon="mdi-delete-alert"
+          :disabled="!resetPreview.count"
+          :loading="resetCommitting"
+          @click="onResetCommit"
+        >Delete {{ resetPreview.count }} content tag(s) +
+          {{ resetPreview.applications }} application(s)</v-btn>
+      </div>
     </v-card-text>
   </v-card>
 </template>
@@ -105,6 +152,9 @@ const committing = ref(false)
 const kindPreview = ref(null)
 const loadingKindPreview = ref(false)
 const kindCommitting = ref(false)
+const resetPreview = ref(null)
+const loadingResetPreview = ref(false)
+const resetCommitting = ref(false)
 
 async function onPreview() {
   loadingPreview.value = true
@@ -143,6 +193,25 @@ async function onKindCommit() {
     kindCommitting.value = false
   }
 }
+
+async function onResetPreview() {
+  loadingResetPreview.value = true
+  try {
+    resetPreview.value = await store.resetContentTagging({ dryRun: true })
+  } finally {
+    loadingResetPreview.value = false
+  }
+}
+
+async function onResetCommit() {
+  resetCommitting.value = true
+  try {
+    await store.resetContentTagging({ dryRun: false })
+    resetPreview.value = { count: 0, by_kind: {}, applications: 0, sample_names: [] }
+  } finally {
+    resetCommitting.value = false
+  }
+}
 </script>
 
 <style scoped>

frontend/src/stores/admin.js

@@ -127,6 +127,21 @@ export const useAdminStore = defineStore('admin', () => {
     }
   }
 
+  // Destructive: deletes ALL general + character tags so the operator can
+  // re-tag from scratch via auto-suggest. fandom + series preserved.
+  async function resetContentTagging({ dryRun = true } = {}) {
+    lastError.value = null
+    try {
+      return await api.post(
+        '/api/admin/tags/reset-content',
+        { body: { dry_run: dryRun } },
+      )
+    } catch (e) {
+      lastError.value = e.message
+      throw e
+    }
+  }
+
   // --- Task progress polling (taps FC-3i activity dashboard) --------
 
   /**
@@ -162,6 +177,7 @@ export const useAdminStore = defineStore('admin', () => {
     tagUsageCount,
     pruneUnusedTags,
     purgeLegacyTags,
+    resetContentTagging,
     pollTaskUntilDone,
   }
 })