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>

migrations/000094_add_point_payment_method.down.sql

@@ -0,0 +1,16 @@
+ALTER TABLE payments
+    DROP CONSTRAINT IF EXISTS chk_payments_point_pair,
+    DROP COLUMN IF EXISTS point_value,
+    DROP COLUMN IF EXISTS points_used;
+
+DROP TRIGGER IF EXISTS trigger_create_point_payment_method ON organizations;
+DROP FUNCTION IF EXISTS create_point_payment_method();
+
+-- Fails if an EnakPoint method has been used by a payment, which is the point: those
+-- payments would lose their method.
+DELETE FROM payment_methods WHERE type = 'point';
+DROP INDEX IF EXISTS uq_payment_methods_point_per_organization;
+
+ALTER TABLE payment_methods DROP CONSTRAINT IF EXISTS payment_methods_type_check;
+ALTER TABLE payment_methods ADD CONSTRAINT payment_methods_type_check
+    CHECK (type IN ('cash', 'card', 'digital_wallet'));