ui: add the missing label for the backup resource limit

[stag7824]

Description

The Resource Limits tab (account, domain and project) shows the backup row labelled with the
raw translation key label.maxbackup instead of a readable name.

ResourceLimitTab.vue derives the label from the resource type name returned by
listResourceLimits:

:label="$t('label.max' + (item.resourcetypename ? item.resourcetypename.replace('_', '') : ''))"

ResourceLimitResponse.setResourceType fills resourcetypename from the enum name, and
Resource.ResourceType declares backup("backup", 12), so the key looked up for that row is
label.maxbackup. That key exists in no locale. en.json has only label.maxbackups, which
is a different string used by the backup schedule form for how many backups to retain
(FormSchedule.vue), and label.maxbackupstorage, which is resource type 13.

i18n is configured with fallbackLocale: 'en' and silentTranslationWarn: true, so the
missing key produces no warning and vue-i18n renders the key itself.

The same key is used for the validation message when a tagged limit exceeds its untagged
limit, so that error currently reads label.maxbackup too:

this.$t('message.update.resource.limit.max.untagged.error')
    .replace('%x', this.$t('label.max' + this.resourceTypeIdNames[resourcetype].replace('_', '')))

Adding the key is enough; nothing else changes. Backup is the only one of the 17
Resource.ResourceType values whose label was missing — the other 16 already resolve.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Feature/Enhancement Scale or Bug Severity

Bug Severity

• Trivial

How Has This Been Tested?

Resolved every Resource.ResourceType through the same expression the template uses, against
the real en.json before and after the change ($t returns the key when it is missing):

type                before                     after
backup              label.maxbackup            Max. Backups   <-- fixed

unchanged types: 16 of 17

So the one row that was broken now resolves, and no other label moves. en.json still parses
(4177 keys).

Found while looking at #13944. That report attributes the problem to resource types 15
(object_storage) and 16 (gpu), but label.maxobjectstorage and label.maxgpu are both
present and resolve correctly, so I do not think this change closes that issue — I have left
a note there with what I found.