Photo: save path, is-local, content bytes, metadata and EXIF getters (closes #141)

A Photo now has savePath and isLocal, which look only at the disk;
content() and exif(), which may download the original; and modifiedAt,
hash, fileSize and year. PhotoRecord gains modifiedAt, hash and fileSize.
The JPEG EXIF scan moves from metadata-backup.ts to the new src/exif.ts,
so the read surface does not import the backup command.

Model: opus-5-5

test/library/records.test.ts

@@ -159,6 +159,8 @@ describe("deriveRecords: photo mapping", () => {
         expect("width" in rec).toBe(false);
         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", () => {