feat: build Go-based SearXNG-compatible search service

Implement an API-first Go rewrite with local engine adapters, upstream fallback, and Nix-based tooling so searches can run without matching the original UI while preserving response compatibility.

Made-with: Cursor

internal/engines/http_mock_test.go

@@ -0,0 +1,26 @@
+package engines
+
+import (
+	"io"
+	"net/http"
+	"strings"
+)
+
+type roundTripperFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripperFunc) RoundTrip(r *http.Request) (*http.Response, error) {
+	return f(r)
+}
+
+func httpResponse(status int, body string, contentType string) *http.Response {
+	h := make(http.Header)
+	if contentType != "" {
+		h.Set("Content-Type", contentType)
+	}
+	return &http.Response{
+		StatusCode: status,
+		Header:     h,
+		Body:       io.NopCloser(strings.NewReader(body)),
+	}
+}
+