Save path ./photos/YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.fileID.ext; download() from the cache first (closes #143)

Originals are saved at `{downloadDirectory}/YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.{fileID}{ext}`. The date is the photo's `takenAt` in local time, and `downloadDirectory` defaults to `./photos`, resolved when the library opens. The old `originals/` layout is gone.

`photo.download()` writes the original to `savePath`. It copies from the cache when the cache holds the original, and fetches otherwise. `lib.backup()` uses the same path and rule, and every album in `collections/` links to it. `isLocal` is true only when the original is at `savePath`.

For a file in several albums, one rule picks the copy everything uses: the most recently synced, with the lowest album ID breaking a tie.

Model: opus-5-5

test/cli/commands.test.ts

@@ -757,7 +757,7 @@ describe("backup", () => {
         expect(code).toBe(1);
         expect(runText).toBe("quak: HTTP 401 from server\n");
         expect(stderr.text).toBe("Starting backup...\nRefreshing library...\n");
-        expect(existsSync(join(dir, "originals"))).toBe(false);
+        expect(existsSync(dir)).toBe(false);
     });
 });