docs(backup): say what account copies and the JSON import can do

Several surfaces promised more than the per-account copies deliver:

- The full-backup export card said it had the same shape as the weekly
  copy. It is a portable export with documents as metadata only; it now
  says so, and that it cannot recreate an account on another server.
- The export page offered to "import from a backup". The JSON import adds
  measurements and mood entries; it is now named that way, and its help
  states the 16 MB and 10,000-entries-per-list limits from the constants
  the route enforces.
- The weekly copy is labelled an account rollback on this server, and the
  backups page says a copy cannot rebuild a lost server. The upload says
  the account must exist here. Two restore refusals pointed to a
  "disaster-recovery" copy; they now name the copies that exist.
- The instance-settings restore option was described as being for
  rebuilding a host.

docs/ops/backup-restore.md is rewritten short and accurate: what each
copy is, the deletion grace window, that database dumps keep a deleted
account until they age out, how to restore one account, and a link to
the disaster-recovery procedure for a lost server.

docs/api/openapi.yaml

@@ -37908,7 +37908,10 @@ components:
         `restoreInstanceSettings` defaults to false: the restore then replaces the snapshot owner's data tables and
         leaves the singleton instance settings — registration, MFA requirement, default locale and timezone, module
         availability, notification and AI configuration, document cap and quota — exactly as they are. Send `true` only
-        when rebuilding a host from a snapshot, because those settings apply to every account on the installation."
+        when this server's instance settings themselves need to go back to the ones in the copy, because those settings
+        apply to every account on the installation. An account copy restores one existing account on this server; it
+        does not rebuild a lost server, which takes a database dump and the encryption keys
+        (`docs/ops/disaster-recovery.md`)."
     CreateSymptomDefinitionRequest:
       type: object
       properties: