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 <hi@josie.lol>
