Open source: add AGPL-3.0 license, NOTICE, and copyright headers

- Add LICENSE (GNU Affero General Public License v3.0)
- Add NOTICE file with SearXNG attribution for borrowed scraping patterns
  in google.py (arc_id, GSA user-agent, HTML selectors) and bing.py
  (PERE pagination, base64 redirect decoding, CAPTCHA detection)
- Add CONTRIBUTING.md with contribution guidelines and license info
- Add SPDX-License-Identifier headers to all Python source files
- Add AGPL-3.0 license badge and License section to README.md

SearXNG is licensed under AGPL-3.0-or-later; this project follows suit
as it incorporates techniques derived from SearXNG's search engines.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/engines/bing.py

@@ -1,5 +1,3 @@
-# SPDX-License-Identifier: AGPL-3.0-or-later
-# Copyright (C) 2026 Junwei Zhao
 # Portions derived from SearXNG <https://github.com/searxng/searxng>
 # Copyright (C) 2021-2024 SearXNG Contributors
 """Bing Search engine — direct scraping with Yahoo fallback on CAPTCHA."""

backend/app/engines/google.py

@@ -1,5 +1,3 @@
-# SPDX-License-Identifier: AGPL-3.0-or-later
-# Copyright (C) 2026 Junwei Zhao
 # Portions derived from SearXNG <https://github.com/searxng/searxng>
 # Copyright (C) 2021-2024 SearXNG Contributors
 """Google Search engine — using SearXNG's async progressive response approach."""