feat: Add referential routes for listing available plans and remove plans endpoint from admin routes

app/routes/admin.py

@@ -171,17 +171,5 @@ def activate_user(user_id: int) -> dict:
         return {"error": f"Failed to activate user: {str(e)}"}, 500
 
 
-@bp.route("/plans")
-@require_auth
-@require_role("admin")
-def list_plans() -> dict:
-    """List all available plans (admin only)."""
-    from app.models.plan import Plan
-    
-    plans = Plan.query.order_by(Plan.id).all()
-    return {
-        "plans": [plan.to_dict() for plan in plans],
-        "total": len(plans)
-    }