quak backup writes each original's EXIF, XMP and dimensions into its JSON (closes #167)

Each file's JSON gains imageMetadata, what extractImageMetadata finds in
the stored original (for a live photo, its image), or the reason the
read failed in imageMetadataError; a failed read fails neither the file
nor the run. A video is not read. An original is read when the run
stores it or when its JSON has neither field; otherwise the field is
carried over from that JSON, so a run does not read every original
again. The hand-built JPEG fixtures move to test/exif-jpeg.ts so the
backup tests can use them.

Judgement call: an original with nothing to record gets imageMetadata
{} instead of no field, so it is not read again on every run.

Model: opus-5-5

TODO.md

@@ -25,6 +25,15 @@ declares one.
 
 # Completed Steps
 
+- 2026-10-06: `quak backup` writes each original's EXIF, XMP and dimensions into
+  the file's JSON as `imageMetadata`, what `backup-metadata --exif` records
+  (issue 167): for a live photo from its image, for a video nothing, and `{}`
+  for an original with none of them. A failed read puts the reason in
+  `imageMetadataError` and fails neither the file nor the run. An original is
+  read when the run stores it or when its JSON has neither field; otherwise the
+  field is taken from that JSON. The hand-built JPEGs moved to
+  `test/exif-jpeg.ts`, beside the HEIC.
+
 - 2026-10-06: `quak backup` retries a failed request for longer than the other
   commands do (issue 165). `src/retry.ts` exports `UNATTENDED_RETRY_OPTIONS`
   beside the unchanged default: 10 attempts, a 1 s base delay and a 60 s cap, so