[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/dev/plugins/ai_summary.rst

@@ -0,0 +1,8 @@
+.. _ai_summary plugin:
+
+==========
+AI summary
+==========
+
+.. automodule:: searx.plugins.ai_summary
+   :members:

docs/dev/plugins/builtins.rst

@@ -7,6 +7,7 @@ Built-in Plugins
 .. toctree::
    :maxdepth: 1
 
+   ai_summary
    calculator
    hash_plugin
    hostnames