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/arxiv_test.go

@@ -0,0 +1,66 @@
+package engines
+
+import (
+	"context"
+	"net/http"
+	"strings"
+	"testing"
+
+	"github.com/ashie/gosearch/internal/contracts"
+)
+
+func TestArxivEngine_Search(t *testing.T) {
+	transport := roundTripperFunc(func(r *http.Request) (*http.Response, error) {
+		if r.Method != http.MethodGet {
+			return httpResponse(http.StatusMethodNotAllowed, "", ""), nil
+		}
+		if r.URL.Host != "export.arxiv.org" || r.URL.Path != "/api/query" {
+			return httpResponse(http.StatusNotFound, "", ""), nil
+		}
+
+		q := r.URL.Query().Get("search_query")
+		if q != "all:quantum" {
+			return httpResponse(http.StatusBadRequest, "", ""), nil
+		}
+
+		atom := `<?xml version="1.0" encoding="UTF-8"?>
+<feed xmlns="http://www.w3.org/2005/Atom">
+  <entry>
+    <title>Quantum Test</title>
+    <id>http://arxiv.org/abs/1234.5678</id>
+    <summary>Abstract here</summary>
+    <published>2024-06-03T00:00:00Z</published>
+  </entry>
+</feed>`
+		return httpResponse(http.StatusOK, atom, "application/atom+xml"), nil
+	})
+
+	client := &http.Client{Transport: transport}
+	engine := &ArxivEngine{client: client}
+
+	resp, err := engine.Search(context.Background(), contracts.SearchRequest{
+		Query:  "quantum",
+		Pageno: 1,
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(resp.Results) != 1 {
+		t.Fatalf("expected 1 result, got %d", len(resp.Results))
+	}
+
+	r := resp.Results[0]
+	if r.Title != "Quantum Test" {
+		t.Fatalf("unexpected title: %q", r.Title)
+	}
+	if r.Content != "Abstract here" {
+		t.Fatalf("unexpected content: %q", r.Content)
+	}
+	if r.URL == nil || !strings.Contains(*r.URL, "1234.5678") {
+		t.Fatalf("unexpected url: %v", r.URL)
+	}
+	if r.Pubdate == nil || !strings.Contains(*r.Pubdate, "2024-06-03") {
+		t.Fatalf("expected pubdate around 2024-06-03, got %v", r.Pubdate)
+	}
+}
+