Decrypt and persist all file metadata layers

Extends RawEnteFile and EnteFile with optional magicMetadata and
pubMagicMetadata fields. Both are secretstream blobs under the file
key, decrypted to arbitrary JSON (Record<string, unknown>).

pubMagicMetadata carries ML-derived data from the Ente clients:
camera make/model, image dimensions, datetime with timezone offset,
and (when present) captions, editedName, face labels, keywords.

magicMetadata carries private mutable fields like visibility.

Backup now writes per-file JSON at originals/<fileID>.json containing
all three metadata layers (basic + magic + pubMagic).

Live-tested: all 11 files in the dev account have pubMagicMetadata
with SONY DSC-RX1RM3 camera info and 3000x2000 dimensions.

src/backup.ts

@@ -116,6 +116,24 @@ export const runBackup = async (
                 }
             }
 
+            // Write per-file metadata JSON alongside the original
+            const metaJsonPath = join(originalsDir, `${file.id}.json`);
+            if (!existsSync(metaJsonPath)) {
+                const fileMeta: Record<string, unknown> = {
+                    id: file.id,
+                    collectionID: file.collectionID,
+                    ownerID: file.ownerID,
+                    metadata: file.metadata,
+                };
+                if (file.magicMetadata) {
+                    fileMeta.magicMetadata = file.magicMetadata;
+                }
+                if (file.pubMagicMetadata) {
+                    fileMeta.pubMagicMetadata = file.pubMagicMetadata;
+                }
+                writeFileSync(metaJsonPath, JSON.stringify(fileMeta, null, 2));
+            }
+
             if (!existsSync(linkPath) && existsSync(origPath)) {
                 const target = relative(colDir, origPath);
                 symlinkSync(target, linkPath);