Fix: Let existing members generate a new invite when a new application is approved

Description

Previously, the system allowed only one invite per user in total. The business requirement is that old members can submit new applications, and when a new application is approved they must be able to generate an invite link again. So we need to allow one invite per user per approved application: an old member who already generated an invite for a past approval should be able to generate a second invite when a new application is approved.

Changes required

User can generate a Discord invite only when they have an approved application; if there’s no approved application (or no application context), they get a clear “not allowed” style message.
Limit is one invite per user per approved application: same user can generate another invite when they get a new application approved (e.g. existing member gets a second invite after a new approval).
Super users can generate an invite on behalf of a user/application when they provide the relevant application (and role) details.
When a user already has an invite for that application, the system responds with a clear “invite already exists” style message instead of creating a duplicate.