#5507 "Alert emails through the outbox, gathered for 2 minutes" (milestone 501). Replaces the in-memory core/alert_burst.py. - outbox.hold_alert: an alert joins each recipient's held alert email, or opens one due 2 minutes later (System 156 ruling). A Postgres advisory lock keeps two alerts at once from opening two windows for one reader. - email_outbox_events.alert_json (migration 0043) keeps the alert as it fired, so the email written when the window closes says what it said. - write_held writes it then, in the reader's zone: the usual alert email for one alert, the list for several (outbox.alert_words). - after_settle records on each AlertEvent, once every email carrying it has settled: email_sent, or email_error naming each reader it didn't reach and the server's reason. - A held email survives a restart and goes out when its window closes. - A refusal of a message's one recipient no longer repeats the address. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>