fix: implement persistent MongoDB connections and resolve race conditions

- Replace connect/disconnect pattern with persistent connection pool
- Add explicit database initialization on server startup
- Remove all dbDisconnect() calls from API endpoints to prevent race conditions
- Fix MongoNotConnectedError when scheduler runs concurrently with API requests
- Add connection pooling with proper MongoDB driver options
- Add safety check for recipes array in favorites utility

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/routes/api/cospend/debts/+server.ts

@@ -1,7 +1,7 @@
 import type { RequestHandler } from '@sveltejs/kit';
 import { PaymentSplit } from '../../../../models/PaymentSplit';
 import { Payment } from '../../../../models/Payment';
-import { dbConnect, dbDisconnect } from '../../../../utils/db';
+import { dbConnect } from '../../../../utils/db';
 import { error, json } from '@sveltejs/kit';
 
 interface DebtSummary {
@@ -105,6 +105,6 @@ export const GET: RequestHandler = async ({ locals }) => {
     console.error('Error calculating debt breakdown:', e);
     throw error(500, 'Failed to calculate debt breakdown');
   } finally {
-    await dbDisconnect();
+    // Connection will be reused
   }
 };