feat(tags): legacy-tag purge also catches source:* general tags

Operator-confirmed 2026-05-28: the BlenderKnight:* tags are `archive`
kind (caught by the kind purge), but the `source:patreon`-style tags
are IR's old `source` kind that fell back to `general` during migration
(FC's enum has no `source` kind) — so they can't be matched by kind.

Broadened the purge to a two-rule match and renamed it for accuracy
(all dev-only, unreleased):
- cleanup_service.purge_tags_by_kind → purge_legacy_tags. Predicate is
  now `kind IN (archive, post, artist) OR name LIKE 'source:%'`
  (LEGACY_NAME_PREFIXES). Preview classifies each row into by_kind OR
  by_prefix (source:* counts once under the prefix bucket regardless of
  its general kind).
- endpoint /tags/purge-retired-kinds → /tags/purge-legacy. dry-run
  returns by_kind + by_prefix + count + sample.
- store purgeRetiredKindTags → purgeLegacyTags.
- Tag Maintenance card copy + breakdown updated to show both buckets;
  button reads "Preview/Delete legacy tags".

Tests updated + extended: dry-run reports by_kind {archive,post,artist}
AND by_prefix {source:*}, plain general/character tags survive; commit
deletes both the kind-matched and source:*-matched rows and leaves the
rest.

frontend/src/stores/admin.js

@@ -114,11 +114,11 @@ export const useAdminStore = defineStore('admin', () => {
     }
   }
 
-  async function purgeRetiredKindTags({ dryRun = true } = {}) {
+  async function purgeLegacyTags({ dryRun = true } = {}) {
     lastError.value = null
     try {
       return await api.post(
-        '/api/admin/tags/purge-retired-kinds',
+        '/api/admin/tags/purge-legacy',
         { body: { dry_run: dryRun } },
       )
     } catch (e) {
@@ -161,7 +161,7 @@ export const useAdminStore = defineStore('admin', () => {
     mergeTags,
     tagUsageCount,
     pruneUnusedTags,
-    purgeRetiredKindTags,
+    purgeLegacyTags,
     pollTaskUntilDone,
   }
 })