Add quak backup-metadata: dump all decrypted metadata to plain JSON

New command: quak backup-metadata <dir>

Dumps every piece of decrypted account metadata into a directory tree
of plain JSON files without downloading any file content. Layout:

    <dir>/
        account.json                    { email, userID }
        collections/
            <id>-<name>/
                _collection.json        { id, name, type, pubMagicMetadata?, ... }
                <fileID>.json           { id, metadata, magicMetadata?, pubMagicMetadata? }

Also adds collection-level magic metadata decryption (magicMetadata,
pubMagicMetadata, sharedMagicMetadata) to decryptCollection, which was
previously only done for files. The server sends these for visibility
settings, sort order, cover photo selection, etc.

6 new tests covering: account.json, per-collection dirs with
_collection.json, collection pubMagicMetadata decryption, per-file
JSON with all three metadata layers, graceful handling of files with
no magic metadata, and incremental re-run safety. 116 total.

src/model/types.ts

@@ -15,6 +15,9 @@ export interface Collection {
     type: CollectionType;
     updationTime: Microseconds;
     isShared: boolean;
+    magicMetadata?: Record<string, unknown>;
+    pubMagicMetadata?: Record<string, unknown>;
+    sharedMagicMetadata?: Record<string, unknown>;
 }
 
 export type FileType = "image" | "video" | "livePhoto" | "unknown";
@@ -59,6 +62,9 @@ export interface RawCollection {
     type: string;
     updationTime: number;
     isDeleted?: boolean;
+    magicMetadata?: RawMagicMetadata;
+    pubMagicMetadata?: RawMagicMetadata;
+    sharedMagicMetadata?: RawMagicMetadata;
 }
 
 export interface RawMagicMetadata {