chore: retire ml-backfill-daily beat + the spent purge-legacy action (operator-approved)

- ml-backfill-daily: the CPU tag_and_embed backfill raced the GPU agent's
  daily embed backfill for the same NULL-embedding images at ~100x the cost
  (B1 audit verdict, milestone #124). The backfill TASK stays — the manual
  /api/ml/backfill button remains the deliberate CPU fallback pending B3.
- purge-legacy: one-time IR-migration cleanup, dry-run verified 0 targets on
  the live library before removal (A2 audit, milestone #123). Fully retired
  per rule 22: tile, store action, route, service fn, tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

frontend/src/stores/admin.js

@@ -101,10 +101,6 @@ export const useAdminStore = defineStore('admin', () => {
     })
   }
 
-  function purgeLegacyTags(opts = {}) {
-    return _dryRunPost('/api/admin/tags/purge-legacy', opts)
-  }
-
   // Destructive: deletes ALL general + character tags so the operator can
   // re-tag from scratch via auto-suggest. fandom + series preserved.
   function resetContentTagging(opts = {}) {
@@ -154,7 +150,6 @@ export const useAdminStore = defineStore('admin', () => {
     pruneUnusedTags,
     pruneBarePosts,
     reconcileDuplicatePosts,
-    purgeLegacyTags,
     resetContentTagging,
     normalizeTags,
     pollTaskUntilDone,