Write each backed-up original once and skip the precache (closes #106)

An original fetched for a backup is now written by the download writer
straight into the backup's originals/, and the content cache records it
there instead of keeping its own copy. One the cache already held is
still copied. `quak backup` opens its library with the thumbnail and
originals precache off, as the one-shot commands do.

Model: opus-5-5

test/cli/backup.test.ts

@@ -562,11 +562,39 @@ describe("lib.backup", () => {
         lib.close();
     });
 
-    it("fsyncs a copied original before the rename and its directory after", async () => {
+    it("fetches each original once and writes it only into the backup", async () => {
+        // A backup of a 500 GB account must write 500 GB, not a copy in the
+        // cache as well: an original fetched for the backup goes straight
+        // into its originals/, and the cache records it there.
+        const source = stubSource();
+        const lib = await openLibrary(source);
+        const outDir = join(root, "backup");
+
+        const result = await lib.backup({ downloadDirectory: outDir });
+
+        expect(result.downloaded).toBe(3);
+        expect(source.originalCalls).toBe(3);
+        expect(readdirSync(join(root, "cache", "originals"))).toEqual([]);
+        const stored = readdirSync(join(outDir, "originals")).filter(
+            (name) => !name.endsWith(".json"),
+        );
+        expect(stored.sort()).toEqual(["100.jpg", "101.jpg", "200.png"]);
+        // The cache counts the backup's copy as present: reading the
+        // original afterwards fetches nothing and answers with that copy.
+        const read = await lib.photos.byID({ fileID: 100 })!.original();
+        expect(read.path).toBe(join(outDir, "originals", "100.jpg"));
+        expect(source.originalCalls).toBe(3);
+        await lib.close();
+    });
+
+    it("fsyncs an original copied from the cache before the rename and its directory after", async () => {
         const lib = await openLibrary(stubSource());
         const outDir = join(root, "backup");
         const originals = join(outDir, "originals");
         const dest = join(originals, "100.jpg");
+        // Only an original already in the cache is copied into the backup;
+        // one fetched for the backup is written there by the download writer.
+        await lib.photos.byID({ fileID: 100 })!.original();
         fsEvents.length = 0;
 
         await lib.backup({ downloadDirectory: outDir });