Attachments upload direct-to-storage on attach, so an abandoned attach (removed,
cancelled, tab closed) would linger forever. Add IiosMediaObject: a row is recorded
when bytes land (MediaService.put); a scheduled sweepOrphans() reaps objects past a
grace window (IIOS_MEDIA_ORPHAN_GRACE_MS, default 24h) that no IiosMessagePart
references (derived live — no flag to drift). Storage delete via the StoragePort;
best-effort + idempotent. Gated on IIOS_MEDIA_GC_INTERVAL_MS (off by default).
Migration 20260722193958_media_object_tracking. 10 media tests green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
