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/config/config.go

@@ -9,7 +9,7 @@ import (
 	"github.com/BurntSushi/toml"
 )
 
-// Config is the top-level configuration for the gosearch service.
+// Config is the top-level configuration for the kafka service.
 type Config struct {
 	Server        ServerConfig         `toml:"server"`
 	Upstream      UpstreamConfig       `toml:"upstream"`