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

test/library/records.test.ts

@@ -160,7 +160,6 @@ describe("deriveRecords: photo mapping", () => {
         expect("height" in rec).toBe(false);
         expect("latitude" in rec).toBe(false);
         expect("hash" in rec).toBe(false);
-        expect("fileSize" in rec).toBe(false);
     });
 
     it("reads archived and hidden from private magicMetadata.visibility", () => {