Collapse /alerts/inbox, /alerts/all, /alerts/history into a single
filterable inbox. Drop ACKNOWLEDGED from AlertState; add read_at and
deleted_at as orthogonal timestamp flags. Retire per-user alert_reads
tracking. Add Silence-rule and Delete row/bulk actions.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>