[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>

searx/settings_defaults.py

@@ -13,6 +13,7 @@ from os.path import dirname, abspath
 import msgspec
 
 from typing_extensions import override
+from .ai_summary import SettingsAISummary
 from .brand import SettingsBrand
 from .sxng_locales import sxng_locales
 from ._settings import SettingsPref
@@ -267,6 +268,7 @@ SCHEMA: dict[str, t.Any] = {
         'networks': {},
     },
     'plugins': SettingsValue(dict, {}),
+    'ai_summary': SettingsAISummary,
     'categories_as_tabs': SettingsValue(dict, CATEGORIES_AS_TABS),
     'engines': SettingsValue(list, []),
     'doi_resolvers': {},