payments/db: split migration queries into dedicated SQLMigrationQueries interface

diff --git a/payments/db/migration1/migration_external_test.go b/payments/db/migration1/migration_external_test.go
index a5ddc42..85f8289 100644
--- a/payments/db/migration1/migration_external_test.go
+++ b/payments/db/migration1/migration_external_test.go
@@ -67,8 +67,14 @@ func TestMigrationWithExternalDB(t *testing.T) {
 		// Run migration in a transaction.
 		err := sqlStore.db.ExecTx(
 			ctx, sqldb.WriteTxOpt(), func(tx SQLQueries) error {
+				migTx, ok := tx.(SQLMigrationQueries)
+				if !ok {
+					return fmt.Errorf("db does not " +
+						"implement SQLMigrationQueries")
+				}
+
 				return MigratePaymentsKVToSQL(
-					ctx, kvBackend, tx, &SQLStoreConfig{
+					ctx, kvBackend, migTx, &SQLStoreConfig{
 						QueryCfg: sqlStore.cfg.QueryCfg,
 					},
 				)
diff --git a/payments/db/migration1/migration_validation.go b/payments/db/migration1/migration_validation.go
index e6f8bdd..b2192e9 100644
--- a/payments/db/migration1/migration_validation.go
+++ b/payments/db/migration1/migration_validation.go
@@ -29,7 +29,7 @@ type migratedPaymentRef struct {
 // with their SQL counterparts. If a structural mismatch is detected, a full
 // deep comparison is performed to produce a detailed diff for debugging.
 func validateMigratedPaymentBatch(ctx context.Context,
-	kvBackend kvdb.Backend, sqlDB SQLQueries,
+	kvBackend kvdb.Backend, sqlDB SQLMigrationQueries,
 	cfg *SQLStoreConfig, batch []migratedPaymentRef) error {
 
 	if len(batch) == 0 {
diff --git a/payments/db/migration1/sql_migration.go b/payments/db/migration1/sql_migration.go
index 626b25a..b208117 100644
--- a/payments/db/migration1/sql_migration.go
+++ b/payments/db/migration1/sql_migration.go
@@ -37,7 +37,7 @@ type MigrationStats struct {
 // migrated data in batches. Callers are responsible for executing this within
 // a single SQL transaction if atomicity is required.
 func MigratePaymentsKVToSQL(ctx context.Context, kvBackend kvdb.Backend,
-	sqlDB SQLQueries, cfg *SQLStoreConfig) error {
+	sqlDB SQLMigrationQueries, cfg *SQLStoreConfig) error {
 
 	if cfg == nil {
 		return fmt.Errorf("missing SQL store config for migration")
@@ -272,7 +272,7 @@ func normalizeTimeForSQL(t time.Time) time.Time {
 
 // migratePayment migrates a single payment from KV to SQL.
 func migratePayment(ctx context.Context, payment *MPPayment, hash lntypes.Hash,
-	sqlDB SQLQueries, stats *MigrationStats) (int64, error) {
+	sqlDB SQLMigrationQueries, stats *MigrationStats) (int64, error) {
 
 	// Update migration stats based on payment status.
 	switch payment.Status {
@@ -602,7 +602,7 @@ func migrateRouteHop(ctx context.Context,
 // migrateDuplicatePayments migrates duplicate payments into the dedicated
 // payment_duplicates table.
 func migrateDuplicatePayments(ctx context.Context, dupBucket kvdb.RBucket,
-	hash [32]byte, primaryPaymentID int64, sqlDB SQLQueries,
+	hash [32]byte, primaryPaymentID int64, sqlDB SQLMigrationQueries,
 	stats *MigrationStats) error {
 
 	duplicateCount := 0
@@ -660,7 +660,7 @@ func migrateDuplicatePayments(ctx context.Context, dupBucket kvdb.RBucket,
 // given payment hash into payment_duplicates.
 func migrateSingleDuplicatePayment(ctx context.Context, dupBucket kvdb.RBucket,
 	hash [32]byte, primaryPaymentID int64, duplicateSeq uint64,
-	sqlDB SQLQueries) error {
+	sqlDB SQLMigrationQueries) error {
 
 	creationData := dupBucket.Get(duplicatePaymentCreationInfoKey)
 	if creationData == nil {
diff --git a/payments/db/migration1/sql_store.go b/payments/db/migration1/sql_store.go
index 0fe4aa1..2a2c977 100644
--- a/payments/db/migration1/sql_store.go
+++ b/payments/db/migration1/sql_store.go
@@ -88,13 +88,16 @@ type SQLQueries interface {
 	// DeleteFailedAttempts removes all failed HTLCs from the db for a
 	// given payment.
 	DeleteFailedAttempts(ctx context.Context, paymentID int64) error
+}
 
-	/*
-		Migration specific queries.
-
-		These queries are used ONLY for the one-time migration from KV
-		to SQL.
-	*/
+// SQLMigrationQueries extends SQLQueries with the additional queries needed
+// for the one-time migration from KV to SQL. Keeping them in a separate
+// interface makes it clear which code paths are migration-only and prevents
+// the regular store from accidentally depending on them.
+//
+//nolint:ll
+type SQLMigrationQueries interface {
+	SQLQueries
 
 	// FetchPaymentsByIDsMig is a migration-only batch fetch that returns
 	// payment data along with HTLC attempt counts for structural
diff --git a/payments/db/sql_store.go b/payments/db/sql_store.go
index 2c8dde9..1c8ffb5 100644
--- a/payments/db/sql_store.go
+++ b/payments/db/sql_store.go
@@ -89,13 +89,16 @@ type SQLQueries interface {
 	// DeleteFailedAttempts removes all failed HTLCs from the db for a
 	// given payment.
 	DeleteFailedAttempts(ctx context.Context, paymentID int64) error
+}
 
-	/*
-		Migration specific queries.
-
-		These queries are used ONLY for the one-time migration from KV
-		to SQL.
-	*/
+// SQLMigrationQueries extends SQLQueries with the additional queries needed
+// for the one-time migration from KV to SQL. Keeping them in a separate
+// interface makes it clear which code paths are migration-only and prevents
+// the regular store from accidentally depending on them.
+//
+//nolint:ll
+type SQLMigrationQueries interface {
+	SQLQueries
 
 	// FetchPaymentsByIDsMig is a migration-only batch fetch that returns
 	// payment data along with HTLC attempt counts for structural