Add request approval system and audiobook path template

Implements admin approval workflow for user requests with global and per-user auto-approve controls. Adds new request statuses ('awaiting_approval', 'denied'), related API endpoints, and UI for pending approvals. Introduces configurable audiobook organization path template with validation and preview in settings, updates database schema and migrations for new fields.

src/app/api/admin/users/route.ts

@@ -30,6 +30,7 @@ export async function GET(request: NextRequest) {
             createdAt: true,
             updatedAt: true,
             lastLoginAt: true,
+            autoApproveRequests: true,
             _count: {
               select: {
                 requests: true,