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/commands.test.ts

@@ -13,6 +13,7 @@
 import {
     existsSync,
     mkdtempSync,
+    readdirSync,
     readFileSync,
     rmSync,
     statSync,
@@ -427,6 +428,16 @@ describe("backup", () => {
         expect(stdout.text).toBe("");
     });
 
+    // The backup opens its library with the precache off: it fetches the
+    // originals it needs into the backup, and must not also fetch every
+    // thumbnail in the account, or keep originals, in the per-user cache.
+    it("leaves nothing in the cache's originals and thumbnails", async () => {
+        const dir = join(root, "backup");
+        expect(await backupCommand(context(), dir, {})).toBe(0);
+        expect(readdirSync(join(root, "cache", "thumbnails"))).toEqual([]);
+        expect(readdirSync(join(root, "cache", "originals"))).toEqual([]);
+    });
+
     it("exits 1 and lists the file when one download fails", async () => {
         const ctx = context(fakeClient({ failID: 101 }));
         expect(await backupCommand(ctx, join(root, "backup"), {})).toBe(1);