Harden the JPEG EXIF scan against malformed input (closes #11)

The segment scan behind `backup-metadata --exif` now checks every
segment length against the bytes that remain and stops on lengths
under 2, so a truncated or corrupt original can neither throw nor loop.
A malformed or unparseable EXIF segment is recorded as
`imageMetadata.exifError`, and a failure to read the original as
`imageMetadataError` in the per-file JSON, instead of the field being
silently left out. Tests use short hand-built byte arrays.

Model: opus-5-5

test/cli/metadata-backup.test.ts

@@ -621,5 +621,18 @@ describe("quak backup-metadata", () => {
         expect(fileMeta.imageMetadata.format).toBe("jpeg");
         expect(fileMeta.imageMetadata.width).toBe(100);
         expect(fileMeta.imageMetadata.height).toBe(80);
+        expect(fileMeta.imageMetadataError).toBeUndefined();
+
+        // File 200 has no original on the mock server, so extraction fails
+        // and the reason is recorded instead of the field being left out.
+        const workDir = collDirs.find((d) => d.includes("Work"))!;
+        const failedMeta = JSON.parse(
+            readFileSync(
+                join(outDir, "collections", workDir, "200.json"),
+                "utf-8",
+            ),
+        );
+        expect(failedMeta.imageMetadata).toBeUndefined();
+        expect(failedMeta.imageMetadataError).toEqual(expect.any(String));
     });
 });