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

src/library/read.ts

@@ -130,9 +130,10 @@ export class Photo {
     }
 
     // The common EXIF fields of the original, read from `content()`, so this
-    // may download it. Only a JPEG's EXIF is read; any other file gives `{}`,
-    // and a video gives it without fetching anything. Like the other content
-    // methods, it throws when there is no content cache, video or not.
+    // may download it. EXIF is read from any image format exifreader reads,
+    // JPEG and HEIC/HEIF among them; any other file gives `{}`, and a video
+    // gives it without fetching anything. Like the other content methods, it
+    // throws when there is no content cache, video or not.
     async exif(opts?: ContentOptions): Promise<PhotoExif> {
         this.cacheOrThrow();
         if (this.rec.fileType === "video") return {};