cairn — Immich sync

Fixes trash requests that could get stuck permanently. If a trash run was interrupted partway through — server unreachable, connection dropped mid-sync, VPN down — the queued retry could wedge and report "A tag with that name already exists" on every attempt after that, and no amount of retrying could clear it. Every trash run writes a breadcrumb tag to Immich so the run can be reconstructed later, and cairn was creating those tags with an endpoint that rejects duplicates rather than the one that reuses an existing tag. Because a retry reuses its original run's tag, every attempt after the first was refused. Retries now reuse the run's existing tag instead of failing on it, and any request already stuck this way clears itself on the next sync after you update. Run breadcrumbs also nest properly under cairn / v1 / run in Immich's tag list now, instead of appearing as one flat entry.