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/read.test.ts

@@ -209,6 +209,31 @@ describe("lib.photos", () => {
         expect("key" in photo.record()).toBe(false);
     });
 
+    it("byID exposes modifiedAt, hash, fileSize and the year taken", () => {
+        // Mid-July, so the year is 2021 in every time zone.
+        const takenAt = Date.UTC(2021, 6, 15, 12);
+        const records = deriveRecords(
+            [collection(1)],
+            [
+                file(1001, 1, {
+                    metadata: {
+                        title: "IMG.jpg",
+                        fileType: "image",
+                        creationTime: micros(takenAt),
+                        modificationTime: micros(1_700_000_123_456),
+                        hash: "aGFzaA==",
+                    },
+                    file: { decryptionHeader: "aGVhZGVy", size: 2_048_000 },
+                }),
+            ],
+        );
+        const photo = apis(records).photos.byID({ fileID: 1001 })!;
+        expect(photo.modifiedAt).toBe(ms(1_700_000_123_456));
+        expect(photo.hash).toBe("aGFzaA==");
+        expect(photo.fileSize).toBe(2_048_000);
+        expect(photo.year).toBe(2021);
+    });
+
     it("byID returns undefined for an unknown file id", () => {
         const records = deriveRecords([collection(1)], [file(1, 1)]);
         expect(apis(records).photos.byID({ fileID: 999 })).toBeUndefined();