feat(coupons): add the coupons catalog resource

[gastonrey]

Why

Factorial's Customer Portal will offer Hyperline customers a retention discount (e.g. 50% for 1 month). Hyperline only attaches a discount to a subscription from an existing catalog coupon (add_coupon needs a coupon_id), so the backend has to find or create that coupon.

What

• client.coupons with list (take / skip, auto-paginate), get, create, update, delete, on top of BaseResource, with the existing Idempotency-Key and retry behaviour.
• delete accepts Hyperline's empty 204 answer.
• The add_coupon / remove_coupon comment on subscriptions now documents their payloads.

Verification

• bundle exec rspec: 165 examples, 0 failures. bundle exec rubocop: no offenses.
• Request and response shapes checked against Hyperline's OpenAPI spec and a sandbox run (create percent coupon -> 201, delete -> 204).

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