refactor(auth): rename paymentBridge → signupBridge

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.

server/api/helcim/initialize-payment.post.js

@@ -2,7 +2,7 @@ import Member from '../../models/member.js'
 import { loadPublicEvent } from '../../utils/loadEvent.js'
 import { loadPublicSeries } from '../../utils/loadSeries.js'
 import { calculateTicketPrice, calculateSeriesTicketPrice, hasMemberAccess } from '../../utils/tickets.js'
-import { requireAuth, getOptionalMember, getPaymentBridgeMember } from '../../utils/auth.js'
+import { requireAuth, getOptionalMember, getSignupBridgeMember } from '../../utils/auth.js'
 import { initializeHelcimPaySession } from '../../utils/helcim.js'
 
 export default defineEventHandler(async (event) => {
@@ -17,7 +17,7 @@ export default defineEventHandler(async (event) => {
 
     if (!isTicket) {
       if (isMembershipSignup) {
-        const bridgeMember = await getPaymentBridgeMember(event)
+        const bridgeMember = await getSignupBridgeMember(event)
         if (!bridgeMember) {
           await requireAuth(event)
         }