fix(ext): wrap /api/extension/manifest filesystem work in asyncio.to_thread (ruff ASYNC240)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/extension.py

@@ -4,6 +4,7 @@
 
 from __future__ import annotations
 
+import asyncio
 import hashlib
 import re
 from pathlib import Path
@@ -86,18 +87,28 @@ async def quick_add_source():
     return jsonify(result), (201 if result["created_source"] else 200)
 
 
-@extension_bp.route("/manifest", methods=["GET"])
-async def extension_manifest():
+def _read_manifest_sync() -> dict | None:
+    """All the filesystem-touching work for /api/extension/manifest,
+    in a sync helper so the async route can dispatch it via
+    asyncio.to_thread (ASYNC240: no pathlib I/O in async functions)."""
     if not XPI_DIR.is_dir():
-        return jsonify({"installed": False}), 404
+        return None
     xpis = sorted(XPI_DIR.glob("fabledcurator-*.xpi"), key=lambda p: p.stat().st_mtime)
     if not xpis:
-        return jsonify({"installed": False}), 404
+        return None
     latest = xpis[-1]
-    return jsonify({
+    return {
         "installed": True,
         "version": _extract_version(latest.name),
         "xpi_url": f"/extension/{latest.name}",
         "latest_url": "/extension/fabledcurator-latest.xpi",
         "sha256": _sha256(latest),
-    })
+    }
+
+
+@extension_bp.route("/manifest", methods=["GET"])
+async def extension_manifest():
+    info = await asyncio.to_thread(_read_manifest_sync)
+    if info is None:
+        return jsonify({"installed": False}), 404
+    return jsonify(info)