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/cli/metadata-exif.test.ts

@@ -8,10 +8,8 @@
  */
 
 import { describe, expect, it } from "vitest";
-import {
-    extractExifFromJpeg,
-    extractImageMetadata,
-} from "../../src/metadata-backup.js";
+import { extractExifFromJpeg } from "../../src/exif.js";
+import { extractImageMetadata } from "../../src/metadata-backup.js";
 
 const SOI = [0xff, 0xd8]; // start of image
 const SOS = [0xff, 0xda, 0x00, 0x02]; // start of scan, where the scan stops