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/library/content-eviction.test.ts

@@ -142,6 +142,7 @@ const buildCache = (args: {
         pools: new RequestPools(),
         source,
         cacheDirectory: cacheDir,
+        downloadDirectory: join(root, "photos"),
         getFile: (id) => byID.get(id),
         statfs: args.statfs,
         cacheOriginalsMaxBytes: args.cacheOriginalsMaxBytes,