[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/plugins/_core.py

@@ -42,7 +42,7 @@ class PluginInfo:
     description: str
     """Short description of the *answerer*."""
 
-    preference_section: t.Literal["general", "ui", "privacy", "query"] | None = "general"
+    preference_section: t.Literal["general", "ui", "privacy", "query", "ai"] | None = "general"
     """Section (tab/group) in the preferences where this plugin is shown to the
     user.