Download all files as ZIP

Stream every file in this job as a single in-memory ZIP.

Review #39 -- users typically want EPW + DDY + STAT (and any sidecar JSON / CSV diagnostics) in one click; the per-file route forces three separate downloads. This builds the archive in-memory using zipfile rather than touching the filesystem, which keeps the OWASP path-traversal surface zero (we only ever iterate the job's own output directory).