fix(backup): answer an upload too large to store with 413

An uploaded file whose encrypted copy passes the size one stored backup may
have on this host (a fifth of the heap limit) stopped the store with
BackupBlobTooLargeError, which the route let through as a 500 and reported
to the error tracker. It is now a 413 with the numbers and the remedy in the
message and meta.errorCode backup.upload.too_large, and the refusal is
audited like the others. The store rolls back, so nothing is kept.

docs/api/openapi.yaml

@@ -30494,8 +30494,8 @@ components:
                 anamnesis.fact.invalidValue; auth — auth.admin.required, auth.already_authenticated,
                 auth.mfa.code_invalid, auth.missing, auth.refresh.invalid, auth.refresh.reuse, auth.refresh.revoked,
                 auth.scope.insufficient, auth.stepup.required, auth.token.expired, auth.token.invalid; backup —
-                backup.payload.undecryptable, backup.restore.active, backup.section.missing; coach —
-                coach.budget.exceeded, coach.conversation.invalidTitle, coach.conversation.notFound,
+                backup.payload.undecryptable, backup.restore.active, backup.section.missing, backup.upload.too_large;
+                coach — coach.budget.exceeded, coach.conversation.invalidTitle, coach.conversation.notFound,
                 coach.fenced.attachmentConflict, coach.fenced.attachmentLimit, coach.fenced.attachmentNotFound,
                 coach.fenced.attachmentNotIndexed, coach.fenced.attachmentRequired, coach.fenced.attachmentUnavailable,
                 coach.fenced.invalid, coach.request.invalid; coach-prefs — coach-prefs.body.invalid_json,