fix(market): marshal empty merge-plan buckets as [] not null

Nil slices in MergePlan.AutoApply/ReviewRequired/Rejected serialized to
JSON null, causing the admin research panel to crash with
"can't access property 'map', plan.review_required is null". Initialize
the buckets as empty slices so the wire contract is always an array.
Tightened the empty-buckets test to assert the JSON shape.

backend/internal/domain/market/merge.go

@@ -145,8 +145,11 @@ func PlanMerge(m Market, r ResearchResult) MergePlan {
 		}
 	}
 
-	// Assign decisions.
-	var autoApply, reviewRequired, rejected []FieldMerge
+	// Assign decisions. Initialize as empty (not nil) so empty buckets
+	// marshal to [] instead of null — the web UI calls .map() on them.
+	autoApply := []FieldMerge{}
+	reviewRequired := []FieldMerge{}
+	rejected := []FieldMerge{}
 
 	for _, vs := range validated {
 		fm := FieldMerge{

backend/internal/domain/market/merge_test.go

@@ -1,6 +1,8 @@
 package market
 
 import (
+	"encoding/json"
+	"strings"
 	"testing"
 	"time"
 )
@@ -177,6 +179,20 @@ func TestPlanMerge(t *testing.T) {
 		if len(plan.AutoApply) != 0 || len(plan.ReviewRequired) != 0 || len(plan.Rejected) != 0 {
 			t.Error("expected all buckets empty for empty suggestions")
 		}
+
+		// Buckets must marshal to JSON arrays, never null — the web UI calls
+		// .map() on them and crashes on null. Go's encoding/json renders nil
+		// slices as null, so PlanMerge must return non-nil empty slices.
+		raw, err := json.Marshal(plan)
+		if err != nil {
+			t.Fatalf("marshal plan: %v", err)
+		}
+		got := string(raw)
+		for _, key := range []string{`"auto_apply":null`, `"review_required":null`, `"rejected":null`} {
+			if strings.Contains(got, key) {
+				t.Errorf("plan JSON contains %s; expected []. Got: %s", key, got)
+			}
+		}
 	})
 }