feat(loyalty): EnakPoint payment method

Adds the system payment method for paying with EnakPoint
(docs/prd-point-coin.md F9, §8, §10.5, PC-303).

Migration 000094 allows the point type, keeps one per organization with a
partial unique index, creates it for every existing organization, and adds
a trigger that creates it for new ones, as the walk-in customer is. It adds
payments.points_used and point_value. Their CHECK is written so it can
never be NULL: the PRD form, (both NULL) OR (both > 0), is NULL for
points_used with a NULL point_value, which a CHECK lets through, so a
payment could have lost the value a refund depends on. A test caught it.

The API cannot create, delete or retype the EnakPoint method, nor turn
another method into one; that answers 400. Renaming it is allowed. The
method list takes the outlet from ?outlet_id= or the user's outlet and
leaves EnakPoint out when that outlet does not accept it, filtered in the
query so the count stays right. The organization-wide active list is
unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/models/payment_method.go

@@ -51,11 +51,13 @@ type PaymentMethodResponse struct {
 
 type ListPaymentMethodsRequest struct {
 	OrganizationID *uuid.UUID
-	Type           *constants.PaymentMethodType
-	IsActive       *bool
-	Search         string
-	Page           int `validate:"min=1"`
-	Limit          int `validate:"min=1,max=100"`
+	// When set, EnakPoint is left out unless the outlet accepts it (F9).
+	OutletID *uuid.UUID
+	Type     *constants.PaymentMethodType
+	IsActive *bool
+	Search   string
+	Page     int `validate:"min=1"`
+	Limit    int `validate:"min=1,max=100"`
 }
 
 type ListPaymentMethodsResponse struct {