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/engine.go

@@ -0,0 +1,17 @@
+package engines
+
+import (
+	"context"
+
+	"github.com/ashie/gosearch/internal/contracts"
+)
+
+// Engine is a Go-native implementation of a SearXNG engine.
+//
+// Implementations should return a SearchResponse containing only the results
+// for that engine subset; the caller will merge multiple engine responses.
+type Engine interface {
+	Name() string
+	Search(ctx context.Context, req contracts.SearchRequest) (contracts.SearchResponse, error)
+}
+