feat: media duplicate purge, part 1 of 3: persisted purge settings and the purge API

api/pkg/apperr/messages.go

@@ -38,3 +38,7 @@ const ArticleBodyFormatErrorMessage = "The format must be one of: markdown, html
 const APIKeyNotFoundErrorMessage = "API key not found"
 const APIKeyExpiryErrorMessage = "The expiry must be a date in the future."
 const SessionRequiredErrorMessage = "This action needs a real session."
+const OwnerOrAdminRequiredErrorMessage = "Only owners and admins can do this."
+const PurgeConflictErrorMessage = "The library changed, so nothing was purged. Check the new summary and try again."
+const PurgeBusyErrorMessage = "A purge is already running."
+const PurgeIntervalErrorMessage = "Enter a whole number of days between 1 and 365."