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/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,
   }
 })