Keep a Photo's save path after its file leaves; refuse an empty download directory

A Photo now keeps the file its record was made from (the record
projection holds one membership of each file), so savePath and isLocal
still answer after a refresh removes the file.

Library.open rejects an empty downloadDirectory.

A backup clears leftover temp files in every date folder under its
directory, not only in those of the files in its scope.

placeOriginal no longer deletes what is at the save path before copying
a live photo, and the tests that depended on that are removed.

The TODO.md entry for issue 143 describes only the current layout.

Model: opus-5-5

test/library/content-library.test.ts

@@ -388,6 +388,34 @@ describe("Photo save path, local copy, content and EXIF", () => {
         await lib.close();
     });
 
+    it("refuses an empty download directory", async () => {
+        await expect(open({ downloadDirectory: "" })).rejects.toThrow(
+            /downloadDirectory is empty/,
+        );
+    });
+
+    it("keeps a photo's save path after a refresh removes its file", async () => {
+        // The same account, whose album is deleted on the second refresh.
+        class AlbumDeletedClient extends MockClient {
+            override async collectionsSince(): Promise<CollectionsPage> {
+                if (!this.served) return super.collectionsSince();
+                return { collections: [], deleted: [1], cursor: 2 };
+            }
+        }
+        const lib = await open({ client: new AlbumDeletedClient() });
+        const photo = lib.photos.byID({ fileID: 1 })!;
+        await photo.download();
+
+        await lib.fresh();
+
+        expect(lib.photos.byID({ fileID: 1 })).toBeUndefined();
+        expect(photo.savePath).toBe(
+            join(root, "backup", DAY, "2026-03-01.1.jpg"),
+        );
+        expect(photo.isLocal).toBe(true);
+        await lib.close();
+    });
+
     it("has a save path, and is not local, without a content source", async () => {
         const lib = await open({ contentSource: undefined });
         const photo = lib.photos.byID({ fileID: 1 })!;