Add migration idempotency test

Verifies that calling Migrate() twice succeeds without error and that
seed data (2 zones, 12 records) is present exactly once.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/db/migrate_test.go

@@ -0,0 +1,47 @@
+package db
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestMigrateIdempotent(t *testing.T) {
+	dir := t.TempDir()
+	database, err := Open(filepath.Join(dir, "test.db"))
+	if err != nil {
+		t.Fatalf("open db: %v", err)
+	}
+	t.Cleanup(func() { _ = database.Close() })
+
+	if err := database.Migrate(); err != nil {
+		t.Fatalf("first migrate: %v", err)
+	}
+	if err := database.Migrate(); err != nil {
+		t.Fatalf("second migrate: %v", err)
+	}
+
+	// Verify seed data is present exactly once.
+	zones, err := database.ListZones()
+	if err != nil {
+		t.Fatalf("list zones: %v", err)
+	}
+	if len(zones) != 2 {
+		t.Fatalf("got %d zones, want 2", len(zones))
+	}
+
+	records, err := database.ListRecords("svc.mcp.metacircular.net", "", "")
+	if err != nil {
+		t.Fatalf("list records: %v", err)
+	}
+	if len(records) != 8 {
+		t.Fatalf("got %d svc records, want 8", len(records))
+	}
+
+	records, err = database.ListRecords("mcp.metacircular.net", "", "")
+	if err != nil {
+		t.Fatalf("list records: %v", err)
+	}
+	if len(records) != 4 {
+		t.Fatalf("got %d mcp records, want 4", len(records))
+	}
+}