fix(backup): refuse a stored copy that will not open with 422

A `DataBackup` row that fails to decrypt answered 500 on the restore, the
download and the preview. It is not a server fault: either the key that wrote
the copy is no longer in `ENCRYPTION_KEYS` — a rotation that dropped the legacy
entry before the script reported zero rows, the case the rotation runbook warns
about — or the stored bytes are not the ones that were written. Every other
bad-input arm on those routes already answers 4xx, and the 500 also paged the
error reporter as if the process had broken.

All three now answer 422 with `meta.errorCode = backup.payload.undecryptable`
and one shared sentence, so the three routes cannot drift into three wordings
for one condition. The download's second 500 stays: a payload this instance
wrote and cannot parse really is a fault here.

The two routes that open a stored copy move from the unpublished list into the
contract, because the refusal is now a promise an operator can look up rather
than whatever the handler happened to answer. The restore publishes its
`Idempotency-Key` header with them.

src/lib/export/backup-blob.ts

@@ -223,6 +223,28 @@ export async function packBackupBlobStreaming(
   return pieces.join("");
 }
 
+/**
+ * The refusal a caller gets when a stored copy will not open, as one string.
+ *
+ * Two causes, one answer, because the envelope cannot tell them apart and the
+ * operator's next step is the same either way: the key that wrote the copy is
+ * no longer in `ENCRYPTION_KEYS` (a rotation that dropped the legacy entry too
+ * early — the case `docs/ops/encryption-key-rotation.md` warns about), or the
+ * stored bytes are not the bytes that were written. Neither is a server fault,
+ * so neither is a 500: it is bad stored input, refused with 422 like every
+ * other bad input on these routes, and it never reaches the error reporter as
+ * if the process had broken.
+ *
+ * Shared rather than retyped, so the three routes that decrypt a stored copy —
+ * restore, download, summary — cannot drift into three different sentences for
+ * the same condition.
+ */
+export const BACKUP_UNDECRYPTABLE_ERROR =
+  "Backup payload could not be decrypted — either the key that wrote this copy is no longer in ENCRYPTION_KEYS, or the stored copy is not the one that was written. Nothing was changed.";
+
+/** The stable machine-readable half of {@link BACKUP_UNDECRYPTABLE_ERROR}. */
+export const BACKUP_UNDECRYPTABLE_CODE = "backup.payload.undecryptable";
+
 /**
  * A stored `DataBackup.data` string → the backup JSON.
  *