One album or file quak cannot decrypt no longer stops a refresh or a backup (closes #181)

A file or album whose key, metadata or name does not decrypt used to
throw out of the refresh, so quak backup stopped before storing
anything, every run. Client.collectionsSince and filesSince now return
such a row in undecryptable with the reason, the refresh leaves it out,
and the store keeps the list in metadata.json until a later refresh
decrypts or deletes the record. lib.backup() records each such file in
failures.json unless a copy in another album decrypted, in which case it
stores the original from that copy. Each such album is listed in
BackupResult.undecryptableAlbums and printed as one FAILED album line.
quak backup exits 1 while either remains. An album or file that no
longer decrypts loses its album folder, JSON and links; its original
stays.

Model: opus-5-5

src/library/index.ts

@@ -601,6 +601,7 @@ export class Library {
                 refresh: () => this.refreshNow(),
                 listCollections: () => this.store.listCollections(),
                 listFiles: (id) => this.store.listFiles(id),
+                undecryptable: () => this.store.listUndecryptable(),
                 original: (fileID, destination) =>
                     cache!.backupOriginal(fileID, destination),
                 thumbnail: (fileID) => cache!.thumbnail(fileID),
@@ -703,7 +704,10 @@ export class Library {
 
     // Fetch every change since the stored cursor, then commit. All network
     // reads happen before any store mutation, so a fetch that throws leaves the
-    // store untouched and the previous snapshot intact.
+    // store untouched and the previous snapshot intact. A collection or file
+    // that does not decrypt is left out instead, as if deleted, with the reason
+    // kept in the store: the cursor moves past it, so no later refresh sees it
+    // again until the server's copy changes.
     private async refreshOnce(): Promise<void> {
         const page = await this.client.collectionsSince({
             sinceTime: this.store.collectionsSinceTime,
@@ -740,9 +744,17 @@ export class Library {
                 this.store.deleteCollection(id);
                 changed = true;
             }
+            if (this.store.deleteUndecryptable(id)) changed = true;
+        }
+        // A collection left out takes its files with it.
+        for (const { id, error } of page.undecryptable ?? []) {
+            this.store.deleteCollection(id);
+            this.store.putUndecryptable({ collectionID: id, error });
+            changed = true;
         }
         for (const collection of page.collections) {
             this.store.putCollection(collection);
+            this.store.deleteUndecryptable(collection.id);
             changed = true;
         }
 
@@ -752,9 +764,22 @@ export class Library {
                     this.store.deleteFile(collectionID, id);
                     changed = true;
                 }
+                if (this.store.deleteUndecryptable(collectionID, id)) {
+                    changed = true;
+                }
+            }
+            for (const { id, error } of filePage.undecryptable ?? []) {
+                this.store.deleteFile(collectionID, id);
+                this.store.putUndecryptable({
+                    collectionID,
+                    fileID: id,
+                    error,
+                });
+                changed = true;
             }
             for (const f of filePage.files) {
                 this.store.putFile(f);
+                this.store.deleteUndecryptable(collectionID, f.id);
                 changed = true;
             }
         }