[feat] engines: add OpenAlex Works engine (#5102)

- Adds a new engine `searx/engines/openalex.py` that integrates the OpenAlex
  Works API to return scientific paper results using the `paper.html` template.
- Uses the official API (no auth required); supports OpenAlex polite pool via `mailto`.
- Adds developer docs at `docs/dev/engines/online/openalex.rst`.

OpenAlex API reference: https://docs.openalex.org/how-to-use-the-api/api-overview

docs/dev/engines/online/openalex.rst

@@ -0,0 +1,100 @@
+.. _openalex engine:
+
+=========
+OpenAlex
+=========
+
+Overview
+========
+
+The OpenAlex engine integrates the `OpenAlex`_ Works API to return scientific paper
+results using the :origin:`paper.html <searx/templates/simple/result_templates/paper.html>`
+template. It is an "online" JSON engine that uses the official public API and does
+not require an API key.
+
+.. _OpenAlex: https://openalex.org
+.. _OpenAlex API overview: https://docs.openalex.org/how-to-use-the-api/api-overview
+
+Key features
+------------
+
+- Uses the official Works endpoint (JSON)
+- Paging support via ``page`` and ``per-page``
+- Relevance sorting (``sort=relevance_score:desc``)
+- Language filter support (maps SearXNG language to ``filter=language:<iso2>``)
+- Maps fields commonly used in scholarly results: title, authors, abstract
+  (reconstructed from inverted index), journal/venue, publisher, DOI, tags
+  (concepts), PDF/HTML links, pages, volume, issue, published date, and a short
+  citations comment
+- Supports OpenAlex "polite pool" by adding a ``mailto`` parameter
+
+
+Configuration
+=============
+
+Minimal example for :origin:`settings.yml <searx/settings.yml>`:
+
+.. code:: yaml
+
+   - name: openalex
+     engine: openalex
+     shortcut: oa
+     categories: science, scientific publications
+     timeout: 5.0
+     # Recommended by OpenAlex: join the polite pool with an email address
+     mailto: "[email protected]"
+
+Notes
+-----
+
+- The ``mailto`` key is optional but recommended by OpenAlex for better service.
+- Language is inherited from the user's UI language; when it is not ``all``, the
+  engine adds ``filter=language:<iso2>`` (e.g. ``language:fr``). If OpenAlex has
+  few results for that language, you may see fewer items.
+- Results typically include a main link. When the primary landing page from
+  OpenAlex is a DOI resolver, the engine will use that stable link. When an open
+  access link is available, it is exposed via the ``PDF`` and/or ``HTML`` links
+  in the result footer.
+
+
+What is returned
+================
+
+Each result uses the ``paper.html`` template and may include:
+
+- ``title`` and ``content`` (abstract; reconstructed from the inverted index)
+- ``authors`` (display names)
+- ``journal`` (host venue display name) and ``publisher``
+- ``doi`` (normalized to the plain DOI, without the ``https://doi.org/`` prefix)
+- ``tags`` (OpenAlex concepts display names)
+- ``pdf_url`` (Open access PDF if available) and ``html_url`` (landing page)
+- ``publishedDate`` (parsed from ``publication_date``)
+- ``pages``, ``volume``, ``number`` (issue)
+- ``type`` and a brief ``comments`` string with citation count
+
+
+Rate limits & polite pool
+=========================
+
+OpenAlex offers a free public API with generous daily limits. For extra courtesy
+and improved service quality, include a contact email in each request via
+``mailto``. You can set it directly in the engine configuration as shown above.
+See: `OpenAlex API overview`_.
+
+
+Troubleshooting
+===============
+
+- Few or no results in a non-English UI language:
+  Ensure the selected language has sufficient coverage at OpenAlex, or set the
+  UI language to English and retry.
+- Preference changes fail while testing locally:
+  Make sure your ``server.secret_key`` and ``server.base_url`` are set in your
+  instance settings so signed cookies work; see :ref:`settings server`.
+
+
+Implementation
+===============
+
+.. automodule:: searx.engines.openalex
+   :members: