multi: remove payment tombstone

diff --git a/config_builder.go b/config_builder.go
index ef3eff9..1a18f21 100644
--- a/config_builder.go
+++ b/config_builder.go
@@ -1134,6 +1134,16 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 
 				// Set the invoice bucket tombstone to indicate
 				// that the migration has been completed.
+				//
+				// TODO(ziggie): The tombstone is currently
+				// set inside the SQL transaction callback,
+				// which is fragile: if the SQL transaction
+				// is retried (e.g. on a serialization
+				// error), the KV tombstone is written before
+				// the SQL commit is confirmed. Move this to
+				// run after ApplyAllMigrations returns so
+				// the tombstone is only set once the
+				// migration is durably committed.
 				d.logger.Debugf("Setting invoice bucket " +
 					"tombstone")
 
@@ -1173,15 +1183,7 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 						"payments to SQL: %w", err)
 				}
 
-				// Set the payments bucket tombstone to
-				// indicate that the migration has been
-				// completed.
-				d.logger.Debugf("Setting payments bucket " +
-					"tombstone")
-
-				return paymentsdb.SetPaymentsBucketTombstone(
-					dbs.ChanStateDB.Backend,
-				)
+				return nil
 			}
 
 			// Make sure we attach the custom migration function to
@@ -1260,6 +1262,7 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 			graphExecutor, graphDBOptions...,
 		)
 		if err != nil {
+			cleanUp()
 			err = fmt.Errorf("unable to get graph store: %w", err)
 			d.logger.Error(err)
 
@@ -1275,6 +1278,7 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 			baseDB, dbs.ChanStateDB.Backend,
 		)
 		if err != nil {
+			cleanUp()
 			err = fmt.Errorf("unable to get payments store: %w",
 				err)
 
@@ -1286,8 +1290,12 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 		// Check if the invoice bucket tombstone is set. If it is, we
 		// need to return and ask the user switch back to using the
 		// native SQL store.
+		//
+		// NOTE: The invoice bucket tombstone acts as the system-wide
+		// guard against switching back to KV mode.
 		ripInvoices, err := dbs.ChanStateDB.GetInvoiceBucketTombstone()
 		if err != nil {
+			cleanUp()
 			err = fmt.Errorf("unable to check invoice bucket "+
 				"tombstone: %w", err)
 			d.logger.Error(err)
@@ -1295,6 +1303,7 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 			return nil, nil, err
 		}
 		if ripInvoices {
+			cleanUp()
 			err = fmt.Errorf("invoices bucket tombstoned, please " +
 				"switch back to native SQL")
 			d.logger.Error(err)
@@ -1302,33 +1311,14 @@ func (d *DefaultDatabaseBuilder) BuildDatabase(
 			return nil, nil, err
 		}
 
-		// Check if the payments bucket tombstone is set. If it is, we
-		// need to return and ask the user switch back to using the
-		// native SQL store.
-		ripPayments, err := paymentsdb.GetPaymentsBucketTombstone(
-			dbs.ChanStateDB.Backend,
-		)
-		if err != nil {
-			err = fmt.Errorf("unable to check payments bucket "+
-				"tombstone: %w", err)
-			d.logger.Error(err)
-
-			return nil, nil, err
-		}
-		if ripPayments {
-			err = fmt.Errorf("payments bucket tombstoned, please " +
-				"switch back to native SQL")
-			d.logger.Error(err)
-
-			return nil, nil, err
-		}
-
 		dbs.InvoiceDB = dbs.ChanStateDB
 
 		graphStore, err = graphdb.NewKVStore(
 			databaseBackends.GraphDB, graphDBOptions...,
 		)
 		if err != nil {
+			cleanUp()
+
 			return nil, nil, err
 		}
 
diff --git a/payments/db/kv_tombstone.go b/payments/db/kv_tombstone.go
deleted file mode 100644
index 7dc13b9..0000000
--- a/payments/db/kv_tombstone.go
+++ /dev/null
@@ -1,71 +0,0 @@
-package paymentsdb
-
-import (
-	"fmt"
-
-	"github.com/lightningnetwork/lnd/kvdb"
-)
-
-var (
-	// paymentsBucketTombstone is the key used to mark the payments bucket
-	// as permanently closed after a successful migration.
-	paymentsBucketTombstone = []byte("payments-tombstone")
-)
-
-// SetPaymentsBucketTombstone sets the tombstone key in the payments bucket to
-// mark the bucket as permanently closed. This prevents it from being reopened
-// in the future.
-func SetPaymentsBucketTombstone(db kvdb.Backend) error {
-	return kvdb.Update(db, func(tx kvdb.RwTx) error {
-		// Access the top-level payments bucket.
-		payments := tx.ReadWriteBucket(paymentsRootBucket)
-
-		// In case the bucket doesn't exist, because we start
-		// immediately with the native SQL schema, we create it as well
-		// to make sure the user cannot switch back to the KV store.
-		if payments == nil {
-			var err error
-			payments, err = tx.CreateTopLevelBucket(
-				paymentsRootBucket,
-			)
-			if err != nil {
-				return fmt.Errorf("failed to create payments "+
-					"bucket: %w", err)
-			}
-		}
-
-		// Add the tombstone key to the payments bucket.
-		err := payments.Put(paymentsBucketTombstone, []byte("1"))
-		if err != nil {
-			return fmt.Errorf("failed to set tombstone: %w", err)
-		}
-
-		return nil
-	}, func() {})
-}
-
-// GetPaymentsBucketTombstone checks if the tombstone key exists in the payments
-// bucket. It returns true if the tombstone is present and false otherwise.
-func GetPaymentsBucketTombstone(db kvdb.Backend) (bool, error) {
-	var tombstoneExists bool
-
-	err := kvdb.View(db, func(tx kvdb.RTx) error {
-		// Access the top-level payments bucket.
-		payments := tx.ReadBucket(paymentsRootBucket)
-		if payments == nil {
-			tombstoneExists = false
-			return nil
-		}
-
-		// Check if the tombstone key exists.
-		tombstone := payments.Get(paymentsBucketTombstone)
-		tombstoneExists = tombstone != nil
-
-		return nil
-	}, func() {})
-	if err != nil {
-		return false, err
-	}
-
-	return tombstoneExists, nil
-}