Adds pending_email column to users table with idempotent migration.
Implements set_pending_email() and promote_pending_email() for email-change workflow.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
La connexion singleton partagée entre threads causait sqlite3.InterfaceError
lors de requêtes concurrentes sur la page compte.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>