fix(frontend & backend): unsubscribe from brain (#2373)

# Description

Please include a summary of the changes and the related issue. Please
also include relevant motivation and context.

## Checklist before requesting a review

Please delete options that are not relevant.

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented hard-to-understand areas
- [ ] I have ideally added tests that prove my fix is effective or that
my feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged

## Screenshots (if appropriate):

backend/routes/subscription_routes.py

@@ -458,11 +458,7 @@ async def unsubscribe_from_brain_handler(
 
     if brain is None:
         raise HTTPException(status_code=404, detail="Brain not found")
-    if brain.status != "public":
-        raise HTTPException(
-            status_code=403,
-            detail="You cannot subscribe to this brain without invitation",
-        )
+
     # check if user is already subscribed to brain
     user_brain = brain_user_service.get_brain_for_user(current_user.id, brain_id)
     if user_brain is None:

frontend/app/studio/[brainId]/page.tsx

@@ -42,7 +42,7 @@ const BrainsManagement = (): JSX.Element => {
       hidden: !isOwnedByCurrentUser || !brain?.max_files,
     },
     {
-      label: "Delete brain",
+      label: isOwnedByCurrentUser ? "Delete Brain" : "Unsubscribe from Brain",
       color: "dangerous",
       onClick: () => {
         setIsDeleteOrUnsubscribeModalOpened(true);