feat: add Redis caching to cospend API endpoints

Implement comprehensive caching for all cospend routes to improve performance:

Cache Implementation:
- Balance API: 30-minute TTL for user balances and global balances
- Debts API: 15-minute TTL for debt breakdown calculations
- Payments List: 10-minute TTL with pagination support
- Individual Payment: 30-minute TTL for payment details

Cache Invalidation:
- Created invalidateCospendCaches() helper function
- Invalidates user balances, debts, and payment lists on mutations
- Applied to payment create, update, and delete operations
- Applied to recurring payment execution (manual and cron)

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

@@ -3,6 +3,7 @@ import { PaymentSplit } from '../../../../models/PaymentSplit';
 import { Payment } from '../../../../models/Payment'; // Need to import Payment for populate to work
 import { dbConnect } from '../../../../utils/db';
 import { error, json } from '@sveltejs/kit';
+import cache from '$lib/server/cache';
 
 export const GET: RequestHandler = async ({ locals, url }) => {
   const auth = await locals.auth();
@@ -14,9 +15,17 @@ export const GET: RequestHandler = async ({ locals, url }) => {
   const includeAll = url.searchParams.get('all') === 'true';
 
   await dbConnect();
-  
+
   try {
     if (includeAll) {
+      // Try cache first for all balances
+      const cacheKey = 'cospend:balance:all';
+      const cached = await cache.get(cacheKey);
+
+      if (cached) {
+        return json(JSON.parse(cached));
+      }
+
       const allSplits = await PaymentSplit.aggregate([
         {
           $group: {
@@ -44,14 +53,27 @@ export const GET: RequestHandler = async ({ locals, url }) => {
         netBalance: 0
       };
 
-      return json({
+      const result = {
         currentUser: currentUserBalance,
         allBalances: allSplits
-      });
+      };
+
+      // Cache for 30 minutes
+      await cache.set(cacheKey, JSON.stringify(result), 1800);
+
+      return json(result);
 
     } else {
+      // Try cache first for individual user balance
+      const cacheKey = `cospend:balance:${username}`;
+      const cached = await cache.get(cacheKey);
+
+      if (cached) {
+        return json(JSON.parse(cached));
+      }
+
       const userSplits = await PaymentSplit.find({ username }).lean();
-      
+
       // Calculate net balance: negative = you are owed money, positive = you owe money
       const netBalance = userSplits.reduce((sum, split) => sum + split.amount, 0);
 
@@ -78,17 +100,22 @@ export const GET: RequestHandler = async ({ locals, url }) => {
             paymentId: split.paymentId._id,
             username: { $ne: username }
           }).lean();
-          
+
           if (otherSplit) {
             split.otherUser = otherSplit.username;
           }
         }
       }
 
-      return json({
+      const result = {
         netBalance,
         recentSplits
-      });
+      };
+
+      // Cache for 30 minutes
+      await cache.set(cacheKey, JSON.stringify(result), 1800);
+
+      return json(result);
     }
 
   } catch (e) {