exif(): read HEIF/HEIC originals with exifreader (closes #145)

`photo.exif()` and `quak backup-metadata --exif` now read EXIF through `exifreader`. HEIC/HEIF originals get EXIF, including a live photo's image, as do the other formats `exifreader` reads. It replaces `exif-reader` and the hand-written JPEG scan. `PhotoExif` is unchanged.

The `backup-metadata` dump now holds `exifreader`'s tag output, with unnamed tags keyed `undefined-` plus their number. GPS altitude without a reference counts as above sea level. A latitude or longitude without its hemisphere tag, an unreadable text tag, and a date the parser rejects each give no field.

Licence: `exifreader` is MPL-2.0, used unmodified.

Model: opus-5-5

README.md

@@ -224,7 +224,7 @@ quak/
         backup.ts       resilient full-account backup with dedup
         metadata-backup.ts
                         backup-metadata: the metadata quak keeps, as JSON
-        exif.ts         EXIF read from a JPEG's bytes
+        exif.ts         EXIF read from an image's bytes with exifreader
         mldata-fetch.ts fetch + decrypt per-file ML data
         filename.ts     safe file names from server metadata
         errors.ts       error types shared across layers
@@ -495,10 +495,15 @@ Ente's clients name them (`IMG_0001.heic` and `IMG_0001.mov`). With
 `--out PATH`, the image is written to `PATH` and the video beside it, with
 `PATH`'s name and the video's extension; a `PATH` with the video's extension is
 refused. `backup-metadata --exif` (alias `--all`) additionally fetches each
-file's original through the cache and records its XMP metadata and, for a JPEG,
-its EXIF metadata and dimensions. `collections`, `files`, `backup`,
-`helper list-missing-thumbnails` and `helper fix-missing-thumbnails` take
-`--json` for machine-readable output.
+file's original through the cache and records, from it or a live photo's image,
+its XMP metadata, its EXIF metadata and, for a JPEG, its dimensions. EXIF is
+read with [exifreader](https://github.com/mattiasw/ExifReader) from any image
+format it reads, JPEG, HEIC/HEIF, AVIF, PNG and WebP among them. The record's
+`exif` field is exifreader's EXIF tag output: each tag by name, with its
+`value`, `description` and `computed` value. An EXIF block exifreader finds but
+reads no tag from is recorded, base64, as `exifRaw`, with the reason in
+`exifError`. `collections`, `files`, `backup`, `helper list-missing-thumbnails`
+and `helper fix-missing-thumbnails` take `--json` for machine-readable output.
 
 `backup-metadata` fetches ML data in requests of up to 200 files. When a request
 fails, the error is logged, each of its files is written with the reason in an
@@ -720,8 +725,9 @@ Four async methods may download:
   each absent when the file lacks it. GPS values are signed decimal degrees and
   metres. `dateTimeOriginal` is the camera's clock reading held in the `Date`'s
   UTC fields; `offsetTimeOriginal`, when present, is that clock's offset from
-  UTC. Only a JPEG's EXIF is read: any other original gives `{}`, and a video
-  gives `{}` without being downloaded.
+  UTC. EXIF is read from any image format exifreader reads (such as JPEG,
+  HEIC/HEIF, AVIF, PNG, WebP and TIFF), a live photo's image included. Any other
+  original gives `{}`, and a video gives `{}` without being downloaded.
 
 They serve from the on-disk content cache when the bytes are present and
 otherwise fetch through the pools; `original()`, `content()` and `exif()` also