Save originals at photos/YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.fileID.ext; add photo.download() (closes #143)

Each original's save path is now
YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.<fileID><ext> under the library's
download directory, dated by takenAt in the machine's time zone. The
directory defaults to photos/ in the working directory, resolved once at
open. savePath is always a string, with or without a content cache, and
isLocal is true only when the original is there.

photo.download() puts the original at its save path: copied from the
cache when the cache holds it, fetched straight there otherwise.
lib.backup() uses the same code for each file, writes each file's JSON
beside it, and links collections/ to the save paths. The backup has no
originals/ folder.

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);
     });
 });