feat(web-api): add event recording for signup, login, bouncer, and billing actions Add events.rs helper module with record() function that inserts into the events table. Record signup/login in auth callback (using created_at timestamp to detect new accounts), bouncer_create/bouncer_delete in bouncer routes, and plan_upgrade/plan_downgrade in billing webhook. Add last_login_at column migration (sql/005) and chrono dependency for timestamp comparison on signup detection. Signed-off-by: Josephine Pfeiffer