Have photo.download() write to the save path of the Photo's own file copy

Photo.download() now hands the content cache the file copy the Photo
holds, the one savePath and isLocal are dated from, instead of the
fileID the cache then looked up in the store. A Photo held across a
refresh that brings a date edited in Ente is now downloaded to the path
its savePath names. A test covers that case.

Model: opus-5-5

test/library/content-library.test.ts

@@ -466,6 +466,48 @@ describe("Photo save path, local copy, content and EXIF", () => {
         await lib.close();
     });
 
+    it("downloads a photo held across a refresh that edits its date to the save path it names", async () => {
+        // The same account, whose second refresh brings a date edited in Ente.
+        const edited = file(1, 1);
+        edited.updationTime = 2;
+        edited.pubMagicMetadata = {
+            editedTime: new Date(2026, 3, 15, 12).getTime() * 1000,
+        };
+        class DateEditedClient extends MockClient {
+            refreshes = 0;
+            override async collectionsSince(): Promise<CollectionsPage> {
+                this.refreshes++;
+                return {
+                    collections: [
+                        { ...collection(1), updationTime: this.refreshes },
+                    ],
+                    deleted: [],
+                    cursor: this.refreshes,
+                };
+            }
+            override async filesSince(): Promise<FilesPage> {
+                return {
+                    files: [this.refreshes === 1 ? file(1, 1) : edited],
+                    deleted: [],
+                    cursor: this.refreshes,
+                };
+            }
+        }
+        const lib = await open({ client: new DateEditedClient() });
+        const photo = lib.photos.byID({ fileID: 1 })!;
+
+        await lib.fresh();
+        expect(lib.photos.byID({ fileID: 1 })!.takenAt).toBe(
+            new Date(2026, 3, 15, 12).getTime(),
+        );
+
+        const saved = await photo.download();
+        expect(saved.path).toBe(photo.savePath);
+        expect(saved.path).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 })!;