Merge SfCompletionWorkflow and NewCustomerSignupWorkflow into a single
AccountCreationWorkflowService, remove legacy /auth/migrate endpoint
and WhmcsLinkWorkflow, extract shared OTP email pattern into
OtpEmailService, and improve PORTAL_EXISTS redirect UX with email
pre-fill.
- Consolidate signup/ directory services into steps/ (PortalUserCreationService,
WhmcsCleanupService) and new AccountCreationWorkflowService
- Rename WhmcsMigrationWorkflowService to AccountMigrationWorkflowService
- Remove dead code: WhmcsLinkWorkflowService, auth.controller /migrate endpoint
- Extract OtpEmailService from duplicated login/verification OTP email logic
- Pass email query param on PORTAL_EXISTS redirect for login pre-fill
- Delete 1977 lines of legacy code, add ~350 lines of consolidated logic
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