[feat] engines: add seek-ninja general engine (#6217)

Add support for https://seek.ninja (general)

It's very slow because the engine uses Server-side events, that incrementally
send data in their HTTP response [1].

I.e. we wait for the end of the response (7+ seconds), even though the results
data arrives within a few seconds -> it's very slow, because SearXNG wants to
get the full response body before it calls the `response(resp)` method

We could use httpx-sse [2], but I'm not sure how to integrate this into SearXNG
and if it's worth it

[1] https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/
[2] https://github.com/florimondmanca/httpx-sse

searx/settings.yml

@@ -1976,6 +1976,14 @@ engines:
   #       - ...
   #   disabled: true
 
+  - name: seekninja
+    engine: seekninja
+    shortcut: sen
+    # very slow due to its server-side events architecture
+    timeout: 10
+    disabled: true
+    inactive: true
+
   - name: semantic scholar
     engine: semantic_scholar
     shortcut: se