paymentsdb+sqldb: add migration related query

diff --git a/payments/db/sql_store.go b/payments/db/sql_store.go
index 3637a98..7f8f9e6 100644
--- a/payments/db/sql_store.go
+++ b/payments/db/sql_store.go
@@ -87,6 +87,25 @@ 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.
+	*/
+
+	// FetchPaymentsByIDsMig is a migration-only batch fetch that returns
+	// payment data along with HTLC attempt counts for structural
+	// validation.
+	FetchPaymentsByIDsMig(ctx context.Context, paymentIDs []int64) ([]sqlc.FetchPaymentsByIDsMigRow, error)
+
+	// InsertPaymentMig is a migration-only variant of InsertPayment that
+	// allows setting fail_reason when inserting historical payments, since
+	// for real payments they have not failed at creation time and so no
+	// failure reason would exist yet.
+	InsertPaymentMig(ctx context.Context, arg sqlc.InsertPaymentMigParams) (int64, error)
+
 }
 
 // BatchedSQLQueries is a version of the SQLQueries that's capable
diff --git a/sqldb/sqlc/payments.sql.go b/sqldb/sqlc/payments.sql.go
index b9ec314..fe02be4 100644
--- a/sqldb/sqlc/payments.sql.go
+++ b/sqldb/sqlc/payments.sql.go
@@ -597,6 +597,72 @@ func (q *Queries) FetchPaymentsByIDs(ctx context.Context, paymentIds []int64) ([
 	return items, nil
 }
 
+const fetchPaymentsByIDsMig = `-- name: FetchPaymentsByIDsMig :many
+SELECT
+    p.id,
+    p.amount_msat,
+    p.created_at,
+    p.payment_identifier,
+    p.fail_reason,
+    COUNT(ha.id) AS htlc_attempt_count
+FROM payments p
+LEFT JOIN payment_htlc_attempts ha ON ha.payment_id = p.id
+WHERE p.id IN (/*SLICE:payment_ids*/?)
+GROUP BY p.id, p.amount_msat, p.created_at, p.payment_identifier, p.fail_reason
+ORDER BY p.id ASC
+`
+
+type FetchPaymentsByIDsMigRow struct {
+	ID                int64
+	AmountMsat        int64
+	CreatedAt         time.Time
+	PaymentIdentifier []byte
+	FailReason        sql.NullInt32
+	HtlcAttemptCount  int64
+}
+
+// Migration-specific batch fetch that returns payment data along with HTLC
+// attempt counts for structural validation during KV to SQL migration.
+func (q *Queries) FetchPaymentsByIDsMig(ctx context.Context, paymentIds []int64) ([]FetchPaymentsByIDsMigRow, error) {
+	query := fetchPaymentsByIDsMig
+	var queryParams []interface{}
+	if len(paymentIds) > 0 {
+		for _, v := range paymentIds {
+			queryParams = append(queryParams, v)
+		}
+		query = strings.Replace(query, "/*SLICE:payment_ids*/?", makeQueryParams(len(queryParams), len(paymentIds)), 1)
+	} else {
+		query = strings.Replace(query, "/*SLICE:payment_ids*/?", "NULL", 1)
+	}
+	rows, err := q.db.QueryContext(ctx, query, queryParams...)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []FetchPaymentsByIDsMigRow
+	for rows.Next() {
+		var i FetchPaymentsByIDsMigRow
+		if err := rows.Scan(
+			&i.ID,
+			&i.AmountMsat,
+			&i.CreatedAt,
+			&i.PaymentIdentifier,
+			&i.FailReason,
+			&i.HtlcAttemptCount,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Close(); err != nil {
+		return nil, err
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const fetchRouteLevelFirstHopCustomRecords = `-- name: FetchRouteLevelFirstHopCustomRecords :many
 SELECT
     l.id,
@@ -918,6 +984,51 @@ func (q *Queries) InsertPaymentIntent(ctx context.Context, arg InsertPaymentInte
 	return id, err
 }
 
+const insertPaymentMig = `-- name: InsertPaymentMig :one
+/* ─────────────────────────────────────────────
+   Migration-specific queries
+
+   These queries are used ONLY for the one-time migration from KV to SQL.
+   ─────────────────────────────────────────────
+*/
+
+INSERT INTO payments (
+    amount_msat,
+    created_at,
+    payment_identifier,
+    fail_reason)
+VALUES (
+    $1,
+    $2,
+    $3,
+    $4
+)
+RETURNING id
+`
+
+type InsertPaymentMigParams struct {
+	AmountMsat        int64
+	CreatedAt         time.Time
+	PaymentIdentifier []byte
+	FailReason        sql.NullInt32
+}
+
+// Migration-specific payment insert that allows setting fail_reason.
+// Normal InsertPayment forces fail_reason to NULL since new payments
+// aren't failed yet. During migration, we're inserting historical data
+// that may already be failed.
+func (q *Queries) InsertPaymentMig(ctx context.Context, arg InsertPaymentMigParams) (int64, error) {
+	row := q.db.QueryRowContext(ctx, insertPaymentMig,
+		arg.AmountMsat,
+		arg.CreatedAt,
+		arg.PaymentIdentifier,
+		arg.FailReason,
+	)
+	var id int64
+	err := row.Scan(&id)
+	return id, err
+}
+
 const insertRouteHop = `-- name: InsertRouteHop :one
 INSERT INTO payment_route_hops (
     htlc_attempt_index,
diff --git a/sqldb/sqlc/querier.go b/sqldb/sqlc/querier.go
index 3be738b..02f2297 100644
--- a/sqldb/sqlc/querier.go
+++ b/sqldb/sqlc/querier.go
@@ -54,6 +54,9 @@ type Querier interface {
 	// Used to avoid fetching redundant payment data when processing multiple
 	// attempts for the same payment.
 	FetchPaymentsByIDs(ctx context.Context, paymentIds []int64) ([]FetchPaymentsByIDsRow, error)
+	// Migration-specific batch fetch that returns payment data along with HTLC
+	// attempt counts for structural validation during KV to SQL migration.
+	FetchPaymentsByIDsMig(ctx context.Context, paymentIds []int64) ([]FetchPaymentsByIDsMigRow, error)
 	// FetchPendingInvoices returns all invoices in a pending state (open or
 	// accepted). The invoices_state_idx index on the state column makes this a
 	// fast index scan rather than a full table scan.
@@ -178,6 +181,11 @@ type Querier interface {
 	InsertPaymentHopCustomRecord(ctx context.Context, arg InsertPaymentHopCustomRecordParams) error
 	// Insert a payment intent for a given payment and return its ID.
 	InsertPaymentIntent(ctx context.Context, arg InsertPaymentIntentParams) (int64, error)
+	// Migration-specific payment insert that allows setting fail_reason.
+	// Normal InsertPayment forces fail_reason to NULL since new payments
+	// aren't failed yet. During migration, we're inserting historical data
+	// that may already be failed.
+	InsertPaymentMig(ctx context.Context, arg InsertPaymentMigParams) (int64, error)
 	InsertRouteHop(ctx context.Context, arg InsertRouteHopParams) (int64, error)
 	InsertRouteHopAmp(ctx context.Context, arg InsertRouteHopAmpParams) error
 	InsertRouteHopBlinded(ctx context.Context, arg InsertRouteHopBlindedParams) error
diff --git a/sqldb/sqlc/queries/payments.sql b/sqldb/sqlc/queries/payments.sql
index 419f7bf..46f84e4 100644
--- a/sqldb/sqlc/queries/payments.sql
+++ b/sqldb/sqlc/queries/payments.sql
@@ -368,3 +368,44 @@ VALUES (
 
 -- name: FailPayment :execresult
 UPDATE payments SET fail_reason = $1 WHERE payment_identifier = $2;
+
+/* ─────────────────────────────────────────────
+   Migration-specific queries
+
+   These queries are used ONLY for the one-time migration from KV to SQL.
+   ─────────────────────────────────────────────
+*/
+
+-- name: InsertPaymentMig :one
+-- Migration-specific payment insert that allows setting fail_reason.
+-- Normal InsertPayment forces fail_reason to NULL since new payments
+-- aren't failed yet. During migration, we're inserting historical data
+-- that may already be failed.
+INSERT INTO payments (
+    amount_msat,
+    created_at,
+    payment_identifier,
+    fail_reason)
+VALUES (
+    @amount_msat,
+    @created_at,
+    @payment_identifier,
+    @fail_reason
+)
+RETURNING id;
+
+-- name: FetchPaymentsByIDsMig :many
+-- Migration-specific batch fetch that returns payment data along with HTLC
+-- attempt counts for structural validation during KV to SQL migration.
+SELECT
+    p.id,
+    p.amount_msat,
+    p.created_at,
+    p.payment_identifier,
+    p.fail_reason,
+    COUNT(ha.id) AS htlc_attempt_count
+FROM payments p
+LEFT JOIN payment_htlc_attempts ha ON ha.payment_id = p.id
+WHERE p.id IN (sqlc.slice('payment_ids')/*SLICE:payment_ids*/)
+GROUP BY p.id, p.amount_msat, p.created_at, p.payment_identifier, p.fail_reason
+ORDER BY p.id ASC;
\ No newline at end of file