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/engines/github.go

@@ -40,7 +40,7 @@ func (e *GitHubEngine) Search(ctx context.Context, req contracts.SearchRequest)
 	if err != nil {
 		return contracts.SearchResponse{}, err
 	}
-	httpReq.Header.Set("User-Agent", "gosearch/0.1")
+	httpReq.Header.Set("User-Agent", "kafka/0.1")
 	httpReq.Header.Set("Accept", "application/vnd.github.v3+json")
 
 	resp, err := e.client.Do(httpReq)