After commit 90acc35 issued the cookie for $0 signups too, the "payment"
framing was wrong — there's no payment in a $0 signup. The cookie is
about bridging the gap between signup-form submit and email verify, not
about payment specifically.
Changes:
- setPaymentBridgeCookie → setSignupBridgeCookie
- getPaymentBridgeMember → getSignupBridgeMember
- Cookie wire name payment-bridge → signup-bridge
- JWT scope payment_bridge → signup_bridge
Touches both /api/helcim/subscription (signup activation) and
/api/helcim/initialize-payment (paid Helcim checkout) which both consume
the cookie. In-flight signup sessions started before this lands will
need to re-submit the form (cookie name mismatch); cutover hasn't
happened yet, so the only impact is local dev sessions.