Get print job footprint hulls

Trusted convex footprint hulls for a set of print jobs — the true XY outlines the plate-merge packer collides instead of bounding boxes, resolved per JOB on the server: a stored part hull applies only when it was computed from the exact source file the job's build snapshot prints. The browser's merge preview and merged-plate thumbnails must use this (not per-part metadata) so they lay out and draw exactly what the server would commit; the build snapshots needed for that check are not synced to clients. Jobs without a trusted hull are absent from the result.