Photo: drop fileSize, exif() throws without a content cache

Remove fileSize from Photo, PhotoRecord, README and TODO.md: the server's
value is the size of the encrypted file, not the original's.

exif() now checks for the content cache before it returns {} for a video,
so it throws like original(), thumbnail() and content() when the library
has no content source. README names the methods that also serve an
original from the backup, since thumbnail() does not.

New tests: exif() on a JPEG whose EXIF block cannot be parsed returns {},
and exif() on a video throws without a content source.

Model: opus-5-5

README.md

@@ -722,8 +722,9 @@ Four async methods may download:
   UTC. Only a JPEG's EXIF is read: any other original gives `{}`, and a video
   gives `{}` without being downloaded.
 
-They serve from the on-disk content cache, or the backup, when the bytes are
-present and otherwise fetch through the pools; `opts.onProgress` reports
+They serve from the on-disk content cache when the bytes are present and
+otherwise fetch through the pools; `original()`, `content()` and `exif()` also
+serve an original the backup has already stored. `opts.onProgress` reports
 per-file progress. They throw when the library was opened without a content
 source. An original that `content()` or `exif()` downloads lands in the cache,
 which does not make `isLocal` true; only `lib.backup()` does.
@@ -741,8 +742,7 @@ The GUI-facing records hold no key material and no binary, so they survive
 - `PhotoRecord`: `fileID`, `albumIDs`, `title`, `takenAt` and `modifiedAt`
   (milliseconds), `fileType`, optional `caption` / `width` / `height` /
   `latitude` / `longitude`, optional `hash` (the content hash recorded at
-  upload; very old files have none) and `fileSize` (the original's size in
-  bytes, as the server reports it), `isArchived`, `isHidden`, and
+  upload; very old files have none), `isArchived`, `isHidden`, and
   `thumbnailPath` / `originalPath` once the bytes are cached (for a live photo,
   `originalPath` is its image).
 - `AlbumRecord`: `collectionID`, `name`, `type`, `isShared`, `updationTime`, and