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>

backend/app/api/admin.py

@@ -224,6 +224,27 @@ async def tags_purge_legacy():
     return jsonify(result)
 
 
+@admin_bp.route("/tags/reset-content", methods=["POST"])
+async def tags_reset_content():
+    """Tier-A: delete ALL general + character tags (the Camie-suggestable
+    content vocabulary) so the operator can re-tag from scratch via
+    auto-suggest. fandom + series tags + series_page ordering are preserved,
+    and image tagger_predictions are untouched so suggestions repopulate.
+    dry-run preview returns per-kind counts + applications + a sample so the
+    UI shows exactly what'll go before the operator confirms (dry_run=false).
+    Irreversible except via DB backup restore."""
+    from ..services.cleanup_service import reset_content_tagging
+
+    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: reset_content_tagging(sync_sess, dry_run=dry_run)
+        )
+    return jsonify(result)
+
+
 @admin_bp.route("/maintenance/db-stats", methods=["GET"])
 async def db_stats():
     """Per-table bloat readout (pg_stat_user_tables) for the high-churn tables