Add --ml and --exif flags to backup-metadata

--ml fetches face detections and CLIP embeddings from the /files/data/fetch
endpoint (type 'mldata'). Each blob is encrypted with the file's key and
gzipped; we decrypt with decryptBlob, gunzip, and include the parsed JSON
as 'mlData' in the per-file output. Fetched in batches of 200 file IDs.

--exif downloads each file, runs sharp().metadata() to extract image
properties (format, dimensions, color space, orientation), then parses
the raw EXIF buffer with exif-reader for structured tags (lens, ISO,
shutter, aperture, GPS altitude, etc.). Also captures raw IPTC, XMP,
and ICC profile data. Included as 'imageMetadata' in the per-file output.

Without either flag, behavior is unchanged (fast metadata-only dump).

Adds exif-reader 2.0.3 as a runtime dependency.
3 new tests (ML data decrypted, ML data absent when flag not set, EXIF
extraction). 119 total tests, all green.

package.json

@@ -41,6 +41,7 @@
     "dependencies": {
         "commander": "14.0.3",
         "env-paths": "4.0.0",
+        "exif-reader": "2.0.3",
         "fast-srp-hap": "2.0.4",
         "libsodium-wrappers-sumo": "0.8.4",
         "sharp": "0.34.5"