Uploader: a file whose only EXIF time is ModifyDate is trusted as a camera time

The uploader reads `DateTimeOriginal`, then `CreateDate`, then `ModifyDate`. A file that carries only `ModifyDate` is treated as camera-timed: no `EST.` tag, no marker, no `timestamp_issues`. ModifyDate is often the time of a later edit or copy, not the capture, so that row can carry a confidently wrong time that nothing flags. Pre-existing behaviour, surfaced while building #237.

Where: `apps/sparcd-uploader/src/lib/exifTime.ts` and the worker's EXIF read.

Fix: either flag ModifyDate-only files like estimates (source `file-modified` or a new `exif-modify`) or at least show them in the capture-times panel so the operator can override.