One multipart XHR POST per file (real upload-progress events, no new
dependency), concurrency capped at 3 to match the server's memory
envelope, an Idempotency-Key per file so a retried blip cannot
double-store, and a canvas downscale to 3000 px longest edge for raster
photos only — PDFs, Office files, and GIFs upload verbatim. Entries
appear synchronously before any network work; an over-limit file fails
locally against the configured cap while the batch continues; a
duplicate resolves to the existing row and hands its id to the timeline
for a brief highlight.
Plan-Task: W2-T4 (manager half)
