fix: centralize engine panic recovery in resilient layer

A rod panic during SearchImage propagated uncaught into fasthttp and killed the whole process: per-engine recover blocks only covered Search (5 of 6 engines had no recovery on SearchImage), and there was no Fiber
recover middleware.

Regression test: a panicking SearchImage returns 502 engine_internal and the server keeps serving.

core/server_panic_test.go

@@ -0,0 +1,71 @@
+package core
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// FP-1: panics from engine code must be converted to ErrEngineInternal by the
+// central recovery in the resilient layer (invokeEngine) instead of killing
+// the process. SearchImage had no per-engine recovery in 5 of 6 engines.
+func TestInvokeEngineRecoversPanics(t *testing.T) {
+	engine := &engineMock{
+		name:        "google",
+		initialized: true,
+		searchFn: func(_ context.Context, _ Query) ([]SearchResult, error) {
+			panic("rod: page crashed")
+		},
+		imageFn: func(_ context.Context, _ Query) ([]SearchResult, error) {
+			panic("rod: object not found")
+		},
+	}
+
+	for _, isImage := range []bool{false, true} {
+		results, err := invokeEngine(context.Background(), engine, Query{Text: "golang"}, isImage)
+		if results != nil {
+			t.Fatalf("isImage=%v: expected nil results after panic, got %v", isImage, results)
+		}
+		if !errors.Is(err, ErrEngineInternal) {
+			t.Fatalf("isImage=%v: expected ErrEngineInternal, got %v", isImage, err)
+		}
+	}
+}
+
+func TestPanickingSearchImageReturns502AndServerSurvives(t *testing.T) {
+	engine := &engineMock{
+		name:        "google",
+		initialized: true,
+		imageFn: func(_ context.Context, _ Query) ([]SearchResult, error) {
+			panic("rod: page crashed")
+		},
+	}
+
+	opts := DefaultServerOptions()
+	opts.Resilience.Retry.MaxRetries = 0
+	srv := NewServerWithOptions("127.0.0.1", 7130, opts, engine)
+
+	req := httptest.NewRequest(http.MethodGet, "/google/image?text=golang", nil)
+	resp, err := srv.app.Test(req, -1)
+	if err != nil {
+		t.Fatalf("image request failed: %v", err)
+	}
+	if resp.StatusCode != http.StatusBadGateway {
+		t.Fatalf("expected 502 for panicking SearchImage, got %d", resp.StatusCode)
+	}
+	var payload JSONErrorResponse
+	if err := json.NewDecoder(resp.Body).Decode(&payload); err != nil {
+		t.Fatalf("decode error response: %v", err)
+	}
+	if payload.Error != "engine_internal" {
+		t.Fatalf("expected error=engine_internal, got %q", payload.Error)
+	}
+
+	second := request(t, srv, "/google/search?text=golang")
+	if second.StatusCode != http.StatusOK {
+		t.Fatalf("expected server to keep serving after panic, got %d", second.StatusCode)
+	}
+}