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/static/css/kafka.css

@@ -1,4 +1,4 @@
-/* gosearch — clean, minimal search engine CSS */
+/* kafka — clean, minimal search engine CSS */
 /* Inspired by SearXNG's simple theme class conventions */
 
 :root {