fix(backup): export, upload and restore a large account as streams

An account of 1.25 million measurements could not export its data or be
restored from any backup (#1031). Measured in a 1 GB container:

- The full export and the passphrase export built the whole payload and
  its JSON string in memory; the app ran out of heap and restarted.
- The weekly backup's JSON is 662 MB, longer than any string V8 can hold,
  so `unpackBackupBlob` threw on every read. Restore, download and the
  restore preview all answered "could not be decrypted".
- Upload accepted 10 MB, a fraction of a large export.

Every reader and writer of a backup file now streams it:

- `scanBackupJson` reads a backup's bytes once, keeps every section but
  the named bulk arrays, and hands their elements out one at a time.
  `readStreamedBackup` uses it twice: a first pass that validates each
  measurement against the element schema and keeps the rest of the file,
  and a second pass that hands the measurements to the writer in batches.
  Stored copies and off-host objects are authenticated whole on their
  compressed ciphertext before any plaintext is read; only the
  decompression is streamed.
- Restore writes the measurements with one `INSERT … FROM unnest` per
  batch. Prisma caches a query plan per `createMany` row count, and one
  1 000-row plan kept about 145 MB alive, which alone put the restore past
  the heap limit. The transaction budget grows with the measurement count.
  The awards and ECG sections are handed the ids they reference that were
  written, not every written id.
- The export routes write the file into the response as it is produced.
  The passphrase export spools its ciphertext to a temporary file because
  the HLX1 format puts the tag first; the format is unchanged, so existing
  archives open as before.
- Upload accepts the raw body, JSON or gzip, up to the 512 MB request
  ceiling, checks it as it arrives and stores it in the same pass; the admin
  page compresses the file in the browser before sending it. Multipart stays
  for small files.
- Download streams the stored JSON; the preview counts measurements from
  the stream. The monthly restore drill and `scripts/restore-backup.ts`
  read the off-host object the same way; the script can write `.json.gz`.

On the seeded account in a 1 GB container: export 25 s at 389 MB peak,
passphrase export 25 s, both uploads 19 s, restore of the weekly backup
94 s at 554 MB peak, and export, upload and restore round-trip to exactly
the same rows. The encrypted-column guard now also recognises the raw SQL
statement that writes `DataBackup.data`.

scripts/restore-backup.ts

@@ -15,12 +15,23 @@
  *   BACKUP_S3_REGION=auto         \
  *   BACKUP_ENCRYPTION_KEY=<hex64> \
  *   pnpm dlx tsx scripts/restore-backup.ts 2026-05-08/user-clx123.json.enc /tmp/restored.json
+ *
+ * An output name ending in `.gz` writes the JSON compressed, which is the
+ * form to hand to the admin upload for a large account: the JSON of 1.25
+ * million measurements is 662 MB, the compressed file about a tenth of it.
+ * The file is written as a stream in either case and never held whole.
  */
-import { writeFileSync } from "node:fs";
+import { createWriteStream } from "node:fs";
+import { rename } from "node:fs/promises";
+import { PassThrough, Readable } from "node:stream";
+import { pipeline } from "node:stream/promises";
+import { createGzip } from "node:zlib";
+
+import { scanBackupJson } from "@/lib/export/backup-json-scan";
 import {
   loadOffhostConfig,
   getS3Client,
-  decryptBackup,
+  openBackupObject,
   OffhostBackupNotConfiguredError,
 } from "@/lib/jobs/offhost-backup";
 
@@ -98,28 +109,43 @@ async function main() {
     `Downloading s3://${cfg.bucket}/${key} from ${cfg.endpoint} (region=${cfg.region})`,
   );
   const ciphertext = await s3.getObject(key);
-  const plaintext = decryptBackup(ciphertext, cfg.encryptionKey);
+  const source = openBackupObject(ciphertext, cfg.encryptionKey);
 
-  let parsed: unknown;
-  try {
-    parsed = JSON.parse(plaintext);
-  } catch (err) {
-    throw new Error(
-      `Decrypted payload is not valid JSON: ${(err as Error).message}`,
-    );
-  }
-  const body = validateBackupShape(parsed);
+  // Checked first, in one streamed pass that keeps only the small sections,
+  // so nothing is written for an object that is not a backup.
+  const { document, streamedCounts } = await scanBackupJson(source(), {
+    streamKeys: new Set(["measurements", "intakeEvents", "moodEntries"]),
+  });
+  const body = validateBackupShape({
+    ...document,
+    measurements: [],
+    intakeEvents: [],
+    moodEntries: [],
+  });
   if (expectedUserId && body.userId !== expectedUserId) {
     throw new Error(
       `Refusing to write: backup.userId='${body.userId}' does not match --user-id='${expectedUserId}'`,
     );
   }
 
-  // The restored file is decrypted PHI in plaintext. Write it 0o600 so it is
-  // not world-readable on the operator's disk while it sits there.
-  writeFileSync(out, plaintext, { encoding: "utf8", mode: 0o600 });
+  // The restored file is decrypted PHI in plaintext. Written 0o600 so it is
+  // not world-readable on the operator's disk, to a temporary name first so
+  // an interrupted run never leaves a truncated file under the real one.
+  const partial = `${out}.part`;
+  let bytes = 0;
+  const counter = new PassThrough();
+  counter.on("data", (chunk: Buffer) => {
+    bytes += chunk.length;
+  });
+  await pipeline(
+    Readable.from(source()),
+    counter,
+    ...(out.endsWith(".gz") ? [createGzip()] : []),
+    createWriteStream(partial, { mode: 0o600 }),
+  );
+  await rename(partial, out);
   console.log(
-    `Restored userId=${body.userId} (${plaintext.length} bytes) -> ${out}`,
+    `Restored userId=${body.userId} (${bytes} bytes of JSON, ${streamedCounts.measurements ?? 0} measurements) -> ${out}`,
   );
 }