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.

backend/app/api/admin.py

@@ -6,6 +6,7 @@ Five action surfaces:
   DELETE /api/admin/tags/<int:tag_id>              (Tier B)
   POST   /api/admin/tags/<int:dest_id>/merge       (Tier B)
   POST   /api/admin/tags/prune-unused              (Tier A)
+  POST   /api/admin/tags/purge-legacy              (Tier A)
   GET    /api/admin/tags/<int:tag_id>/usage-count  (helper)
 
 Tier-C ops take a dry_run body flag (returns projection inline,
@@ -208,20 +209,21 @@ async def tags_prune_unused():
     return jsonify(result)
 
 
-@admin_bp.route("/tags/purge-retired-kinds", methods=["POST"])
-async def tags_purge_retired_kinds():
-    """Tier-A: delete tags whose kind is a system/legacy kind FC no
-    longer creates (archive / post / artist — the IR-migration leftovers
-    like `BlenderKnight:Hannah_BJ_Loops`). dry-run preview returns
-    per-kind counts + a sample so the UI can show exactly what'll go
-    before the operator confirms with dry_run=false."""
-    from ..services.cleanup_service import purge_tags_by_kind
+@admin_bp.route("/tags/purge-legacy", methods=["POST"])
+async def tags_purge_legacy():
+    """Tier-A: delete legacy IR-migration tags — archive/post/artist
+    kinds (e.g. `BlenderKnight:Hannah_BJ_Loops`) PLUS general tags with
+    a legacy name prefix (`source:*`, from IR's source kind that fell
+    back to general). dry-run preview returns per-kind + per-prefix
+    counts + a sample so the UI shows exactly what'll go before the
+    operator confirms with dry_run=false."""
+    from ..services.cleanup_service import purge_legacy_tags
 
     body = await request.get_json(silent=True) or {}
     dry_run = bool(body.get("dry_run", False))
 
     async with get_session() as session:
         result = await session.run_sync(
-            lambda sync_sess: purge_tags_by_kind(sync_sess, dry_run=dry_run)
+            lambda sync_sess: purge_legacy_tags(sync_sess, dry_run=dry_run)
         )
     return jsonify(result)