[feat] plugin: AI summary of search results via local Ollama server

Add an optional, disabled-by-default plugin that shows an AI generated
summary at the top of the result page, generated by a (local) Ollama
server:

- async: the result page is never delayed; a client plugin streams the
  answer (NDJSON over a new /ai_summary endpoint) into a placeholder
  answer with a typing indicator, collapsed behind a More button, with
  an inline follow-up chat
- trigger: first page of general searches only, skipped when an infobox
  or instant answer already answers the query
- grounding (per-user preference): send the top result snippets as
  context, the model answers from them instead of its own knowledge
- configuration: new AI Summary preferences tab (server URL, model,
  grounding) with instance defaults in a new ai_summary: settings
  section; all three preferences can be locked for public instances

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/admin/settings/index.rst

@@ -25,3 +25,4 @@ Settings
    settings_outgoing
    settings_categories_as_tabs
    settings_plugins
+   settings_ai_summary

docs/admin/settings/settings_ai_summary.rst

@@ -0,0 +1,36 @@
+.. _settings ai_summary:
+
+===============
+``ai_summary:``
+===============
+
+Default configuration of the :ref:`AI summary plugin <ai_summary plugin>`.
+Users configure the Ollama server URL and the model in the *AI Summary* tab of
+their preferences; the values below only act as instance wide defaults.
+
+.. code:: yaml
+
+   ai_summary:
+     base_url: "http://127.0.0.1:11434"
+     model: "llama3.2:3b"
+
+.. attention::
+
+   A user configurable server URL allows any user of the instance to make the
+   SearXNG server send requests to a URL of their choice (`SSRF`_), and each
+   summary is real LLM work.  This plugin is intended for private instances --
+   on a public instance lock the related preferences (:ref:`settings
+   preferences`):
+
+   .. code:: yaml
+
+      preferences:
+        lock:
+          - ai_summary_server
+          - ai_summary_model
+          - ai_summary_grounding
+
+.. _SSRF: https://owasp.org/www-community/attacks/Server_Side_Request_Forgery
+
+.. autoclass:: searx.ai_summary.SettingsAISummary
+   :members: