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

TODO.md

@@ -26,13 +26,13 @@ declares one.
 # Completed Steps
 
 - 2026-10-01: A `Photo` has `savePath`, `isLocal`, `content()`, `exif()`,
-  `modifiedAt`, `hash`, `fileSize` and `year` (issue 141). `savePath` is where
+  `modifiedAt`, `hash` and `year` (issue 141). `savePath` is where
   `lib.backup()` writes the original under the library's download directory, and
   `isLocal` says whether the whole original is there; both look only at the
   disk. `content()` returns the original's bytes and `exif()` the common EXIF
   fields of a JPEG; both may download the original, and `exif()` downloads no
-  video. `PhotoRecord` gains `modifiedAt`, `hash` and `fileSize`, and the JPEG
-  EXIF scan moved to `src/exif.ts`.
+  video. `PhotoRecord` gains `modifiedAt` and `hash`, and the JPEG EXIF scan
+  moved to `src/exif.ts`.
 
 - 2026-09-29: The "Workflow" list at the top of this file now says to branch
   from `next` and open a pull request that targets `next`, that the repository