sqldb: Change payment_intent relationship to payment table

Previously a one(intent)-to-many(payment) relationship it is now
changed to a one-to-one relationship because a payment request
only can have 1 payment related to it. Looking into the future
with BOLT12 offers, the fetched invoice from the offer could be
stored here as well and the relationship would still hold.

sqldb/sqlc/models.go

@@ -211,7 +211,6 @@ type MigrationTracker struct {
 
 type Payment struct {
 	ID                int64
-	IntentID          sql.NullInt64
 	AmountMsat        int64
 	CreatedAt         time.Time
 	PaymentIdentifier []byte
@@ -264,6 +263,7 @@ type PaymentHtlcAttemptResolution struct {
 
 type PaymentIntent struct {
 	ID            int64
+	PaymentID     int64
 	IntentType    int16
 	IntentPayload []byte
 }