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/backup.ts

@@ -4,7 +4,8 @@
 // fails the backup before any file is touched), then, for every file in scope,
 // gets its original bytes onto disk under `downloadDirectory` and rebuilds the
 // derived views (per-file sidecars, per-collection symlink trees,
-// per-collection JSON) from the model. The on-disk layout is the historical one, unchanged:
+// per-collection JSON) from the model. The on-disk layout is the historical
+// one, unchanged:
 //
 //     <downloadDirectory>/
 //         originals/<fileID>.<ext>     the decrypted bytes
@@ -96,8 +97,9 @@ export interface BackupLibrary {
     listCollections(): Collection[];
     listFiles(collectionID: number): EnteFile[];
     // Get an original's bytes onto disk through the content cache/pools,
-    // returning where they landed (the cache, or a prior backup).
-    original(fileID: number): Promise<{ path: string }>;
+    // returning where they landed: `destination` when they were fetched now,
+    // otherwise wherever they already were (the cache, or a prior backup).
+    original(fileID: number, destination: string): Promise<{ path: string }>;
     thumbnail(fileID: number): Promise<{ path: string }>;
 }
 
@@ -424,7 +426,9 @@ export const runBackup = async (
             }
             try {
                 log(`Fetching original ${file.metadata.title} (${fileID})...`);
-                const { path } = await lib.original(fileID);
+                // A fetched original is written straight to `dest`; only one
+                // that was already cached elsewhere is copied.
+                const { path } = await lib.original(fileID, dest);
                 await copyAtomic(path, dest);
                 downloaded++;
             } catch (err) {