feat: Add the backup dump

Collections are enumerated live rather than from a list, so a collection
added later is backed up with no code change. Documents are written as the
raw BSON the driver returned, so Decimal128, ObjectId, DateTime and binary
subtypes survive byte for byte.

backup/mongo_test.go

@@ -0,0 +1,44 @@
+package backup
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"testing"
+	"time"
+
+	"go.mongodb.org/mongo-driver/v2/mongo"
+	"go.mongodb.org/mongo-driver/v2/mongo/options"
+)
+
+// testDB connects to the MongoDB named by MONGO_TEST_URI and returns a client
+// plus a database name unique to this test, dropped when the test ends.
+//
+// Skips rather than fails when the variable is unset: these tests need a real
+// server, and a developer without one should still be able to run the rest of
+// the suite.
+func testDB(t *testing.T) (*mongo.Client, string) {
+	t.Helper()
+	uri := os.Getenv("MONGO_TEST_URI")
+	if uri == "" {
+		t.Skip("MONGO_TEST_URI is not set; skipping tests that need MongoDB")
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	client, err := mongo.Connect(options.Client().ApplyURI(uri))
+	if err != nil {
+		t.Fatalf("connect: %v", err)
+	}
+	if err := client.Ping(ctx, nil); err != nil {
+		t.Fatalf("ping: %v", err)
+	}
+	name := fmt.Sprintf("vantage_test_%d", time.Now().UnixNano())
+	t.Cleanup(func() {
+		c, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+		_ = client.Database(name).Drop(c)
+		_ = client.Disconnect(c)
+	})
+	return client, name
+}