[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.py

@@ -33,6 +33,9 @@ class SettingsPref(msgspec.Struct, kw_only=True, forbid_unknown_fields=True):
             "theme",
             "results_on_new_tab",
             "doi_resolver",
+            "ai_summary_server",
+            "ai_summary_model",
+            "ai_summary_grounding",
             "simple_style",
             "center_alignment",
             "query_in_title",