Preflight over the declared uncompressed sizes (overflow-safe), a file
count cap and a per-user storage quota bound the import before
anything is read; the actual decompressed bytes are still counted
against the same budget while reading. Attachments and backgrounds
now stream through a lazy provider: one zip entry is decompressed
into a private temp file at a time. A failed import deletes the file
blobs it wrote, which were orphaned for every migrator before.