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Add server-side impersonation session management with full audit logging, replacing the frontend-only mock. Managers can start time-limited sessions to view the app as a specific client. Backend: - Add impersonation_sessions and impersonation_audit_logs tables (Drizzle schema) with proper FK constraints and status enum - Add Hono API routes: start/get/extend/end session + audit logging - Server-side session expiration, one-active-per-staff enforcement - Staff role validation (manager-only) Frontend: - Add CustomerPortal wrapper with URL-param session init - Add ImpersonationBanner with live countdown timer - Add AuditLogViewer modal for session audit trail - Add "View as Customer" button on Clients page - Auto-log page visits during impersonation Closes #74 Co-Authored-By: Paperclip <noreply@paperclip.ing>