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/api/routes.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """API routes for Hey Search."""
 
 from __future__ import annotations

backend/app/background.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Background image management — fetch from Unsplash, cache locally."""
 
 from __future__ import annotations

backend/app/bookmarks.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """SQLite-backed bookmarks storage."""
 
 from __future__ import annotations

backend/app/cache.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Optional Redis cache for search results.
 
 When REDIS_URL is not set or Redis is unreachable, all operations

backend/app/engines/base.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Abstract base class for search engines."""
 
 from __future__ import annotations

backend/app/engines/bing.py

@@ -1,3 +1,7 @@
+# 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."""
 
 from __future__ import annotations

backend/app/engines/brave.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Brave Search engine implementation."""
 
 from __future__ import annotations

backend/app/engines/date_utils.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Date extraction utilities for search engine result snippets."""
 
 from __future__ import annotations

backend/app/engines/duckduckgo.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """DuckDuckGo Search engine implementation."""
 
 from __future__ import annotations

backend/app/engines/google.py

@@ -1,3 +1,7 @@
+# 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."""
 
 from __future__ import annotations

backend/app/engines/registry.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Engine registry - manages available search engines."""
 
 from __future__ import annotations

backend/app/excluded.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """SQLite-backed storage for excluded domains."""
 
 from __future__ import annotations

backend/app/history.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Search history – reads from the stats search_events table."""
 
 from __future__ import annotations

backend/app/main.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Hey Search - A metasearch engine."""
 
 import json

backend/app/models.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Result models for search engines."""
 
 from __future__ import annotations

backend/app/search.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Search orchestrator - coordinates searches across multiple engines concurrently."""
 
 from __future__ import annotations

backend/app/settings.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """SQLite-backed application settings."""
 
 from __future__ import annotations

backend/app/stats.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+# Copyright (C) 2026 Junwei Zhao
 """Search analytics – persisted in a local SQLite database."""
 
 from __future__ import annotations