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.

tests/server/api/helcim-subscription.test.js

@@ -15,7 +15,7 @@ vi.mock('../../../server/models/member.js', () => ({
 vi.mock('../../../server/utils/mongoose.js', () => ({ connectDB: vi.fn() }))
 vi.mock('../../../server/utils/auth.js', () => ({
   requireAuth: vi.fn(),
-  getPaymentBridgeMember: vi.fn().mockResolvedValue(null)
+  getSignupBridgeMember: vi.fn().mockResolvedValue(null)
 }))
 vi.mock('../../../server/utils/slack.ts', () => ({
   getSlackService: vi.fn().mockReturnValue(null)