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/index.html

@@ -1,7 +1,7 @@
 {{define "title"}}{{end}}
 {{define "content"}}
 <div class="index">
-  <div class="title"><h1>gosearch</h1></div>
+  <div class="title"><h1>kafka</h1></div>
   <div id="search">
     <form method="GET" action="/search" role="search">
       <input type="text" name="q" id="q" placeholder="Search…" autocomplete="off" autofocus