chore: rename project from gosearch to kafka

A search engine named after a man who proved answers don't exist.

Renamed everywhere user-facing:
- Brand name, UI titles, OpenSearch description, CSS filename
- Docker service name, NixOS module (services.kafka)
- Cache key prefix (kafka:), User-Agent strings (kafka/0.1)
- README, config.example.toml, flake.nix descriptions

Kept unchanged (internal):
- Go module path: github.com/ashie/gosearch
- Git repository URL: git.ashisgreat.xyz/penal-colony/gosearch
- Binary entrypoint: cmd/searxng-go

internal/views/templates/opensearch.xml

@@ -1,12 +1,12 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
-  <ShortName>gosearch</ShortName>
+  <ShortName>kafka</ShortName>
   <Description>A privacy-respecting, open metasearch engine</Description>
   <InputEncoding>UTF-8</InputEncoding>
   <OutputEncoding>UTF-8</OutputEncoding>
-  <LongName>gosearch — Privacy-respecting metasearch</LongName>
+  <LongName>kafka — Privacy-respecting metasearch</LongName>
   <Image width="16" height="16" type="image/svg+xml">/static/img/favicon.svg</Image>
-  <Contact>https://git.ashisgreat.xyz/penal-colony/gosearch</Contact>
+  <Contact>https://git.ashisgreat.xyz/penal-colony/kafka</Contact>
   <Url type="text/html" method="GET" template="{baseUrl}/search?q={searchTerms}&amp;format=html">
     <Param name="pageno" value="{startPage?}" />
     <Param name="language" value="{language?}" />