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

src/cli-commands.ts

@@ -380,10 +380,14 @@ export const backupCommand = async (
     if (!client) return 1;
 
     ctx.stderr.write("Starting backup...\n");
+    // The precache is off: the backup fetches what it needs, and must not
+    // also fill the cache with every thumbnail and the recent originals.
     const lib = await Library.open({
         client,
         downloadDirectory: dir,
         cacheDirectory: ctx.cacheDir,
+        precacheThumbnails: false,
+        precacheOriginals: false,
     });
     try {
         const result = await lib.backup({