paymentsdb: sort FetchInFlightPayments result by sequence number

The SQL implementation collects payments into a map before converting
to a slice, resulting in non-deterministic iteration order due to Go's
intentional map randomisation. Sort the result by SequenceNum to produce
a deterministic, insertion-ordered output.

Note that the current sole caller (resumePayments in router.go) processes
each payment independently, so this ordering does not affect any existing
behaviour.

payments/db/sql_store.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"sort"
 	"strconv"
 	"time"
 
@@ -1060,11 +1061,16 @@ func (s *SQLStore) FetchInFlightPayments(ctx context.Context) ([]*MPPayment,
 			return err
 		}
 
-		// Convert map to slice.
+		// Convert map to slice and sort by sequence number to
+		// produce a deterministic ordering.
 		mpPayments = make([]*MPPayment, 0, len(processedPayments))
 		for _, payment := range processedPayments {
 			mpPayments = append(mpPayments, payment)
 		}
+		sort.Slice(mpPayments, func(i, j int) bool {
+			return mpPayments[i].SequenceNum <
+				mpPayments[j].SequenceNum
+		})
 
 		return nil
 	}, func() {