fix(launch-flow): auto-link /join signups to existing PreRegistration

When a /join submitter's email matches a pending/selected/invited
PreRegistration, mark the pre-reg as accepted and link memberId to the
new Member. Prevents the same person from appearing as both an active
member and an unaccepted pre-registrant. Silent — no email, no UI.

Adds the PreRegistration mock to helcim-customer and free-signup-flow
test suites, since both invoke the customer handler at runtime.

tests/server/api/free-signup-flow.test.js

@@ -20,6 +20,9 @@ vi.mock('../../../server/models/member.js', () => ({
     findOneAndUpdate: vi.fn()
   }
 }))
+vi.mock('../../../server/models/preRegistration.js', () => ({
+  default: { findOne: vi.fn().mockResolvedValue(null), findByIdAndUpdate: vi.fn() }
+}))
 vi.mock('../../../server/utils/mongoose.js', () => ({ connectDB: vi.fn() }))
 vi.mock('../../../server/utils/helcim.js', () => ({
   createHelcimCustomer: vi.fn(),

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

@@ -12,6 +12,9 @@ import { createMockEvent } from '../helpers/createMockEvent.js'
 vi.mock('../../../server/models/member.js', () => ({
   default: { findOne: vi.fn(), create: vi.fn(), findByIdAndUpdate: vi.fn() }
 }))
+vi.mock('../../../server/models/preRegistration.js', () => ({
+  default: { findOne: vi.fn().mockResolvedValue(null), findByIdAndUpdate: vi.fn() }
+}))
 vi.mock('../../../server/utils/mongoose.js', () => ({ connectDB: vi.fn() }))
 vi.mock('../../../server/utils/helcim.js', () => ({
   createHelcimCustomer: vi.fn()