Add `POST /image/exif/date` and `POST /image/exif/date/clear` so an
operator can correct a row whose canonical-date waterfall landed on the
wrong value (camera clock reset, fs_time fallback for a copied-from-
backup file, etc). New `original_date_taken` / `original_date_taken_source`
columns snapshot the prior value on first override so revert is lossless.
The waterfall source set is now `'exif' | 'exiftool' | 'filename' | 'fs_time' | 'manual'`.
The existing `idx_image_exif_date_backfill` partial index already filters
to `date_taken IS NULL OR date_taken_source = 'fs_time'`, so manual rows
are naturally excluded from the per-tick drain — no index change needed.
`ExifMetadata` now exposes `date_taken_source` + originals so a UI can
render "manually set; was X via filename".
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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