Add comprehensive recurring payments system with scheduling

- Add RecurringPayment model with flexible scheduling options
- Implement node-cron based scheduler for payment processing
- Create API endpoints for CRUD operations on recurring payments
- Add recurring payments management UI with create/edit forms
- Integrate scheduler initialization in hooks.server.ts
- Enhance payments/add form with progressive enhancement
- Add recurring payments button to main dashboard
- Improve server-side rendering for better performance

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

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

src/routes/api/cospend/recurring-payments/+server.ts

@@ -0,0 +1,139 @@
+import type { RequestHandler } from '@sveltejs/kit';
+import { RecurringPayment } from '../../../../models/RecurringPayment';
+import { dbConnect, dbDisconnect } from '../../../../utils/db';
+import { error, json } from '@sveltejs/kit';
+import { calculateNextExecutionDate, validateCronExpression } from '../../../../lib/utils/recurring';
+
+export const GET: RequestHandler = async ({ locals, url }) => {
+  const auth = await locals.auth();
+  if (!auth || !auth.user?.nickname) {
+    throw error(401, 'Not logged in');
+  }
+
+  const limit = parseInt(url.searchParams.get('limit') || '50');
+  const offset = parseInt(url.searchParams.get('offset') || '0');
+  const activeOnly = url.searchParams.get('active') === 'true';
+
+  await dbConnect();
+  
+  try {
+    const query: any = {};
+    if (activeOnly) {
+      query.isActive = true;
+    }
+
+    const recurringPayments = await RecurringPayment.find(query)
+      .sort({ nextExecutionDate: 1, createdAt: -1 })
+      .limit(limit)
+      .skip(offset)
+      .lean();
+
+    return json({ recurringPayments });
+  } catch (e) {
+    console.error('Error fetching recurring payments:', e);
+    throw error(500, 'Failed to fetch recurring payments');
+  } finally {
+    await dbDisconnect();
+  }
+};
+
+export const POST: RequestHandler = async ({ request, locals }) => {
+  const auth = await locals.auth();
+  if (!auth || !auth.user?.nickname) {
+    throw error(401, 'Not logged in');
+  }
+
+  const data = await request.json();
+  const { 
+    title, 
+    description, 
+    amount, 
+    paidBy, 
+    category, 
+    splitMethod, 
+    splits, 
+    frequency, 
+    cronExpression,
+    startDate,
+    endDate
+  } = data;
+
+  if (!title || !amount || !paidBy || !splitMethod || !splits || !frequency) {
+    throw error(400, 'Missing required fields');
+  }
+
+  if (amount <= 0) {
+    throw error(400, 'Amount must be positive');
+  }
+
+  if (!['equal', 'full', 'proportional', 'personal_equal'].includes(splitMethod)) {
+    throw error(400, 'Invalid split method');
+  }
+
+  if (!['daily', 'weekly', 'monthly', 'custom'].includes(frequency)) {
+    throw error(400, 'Invalid frequency');
+  }
+
+  if (frequency === 'custom') {
+    if (!cronExpression || !validateCronExpression(cronExpression)) {
+      throw error(400, 'Valid cron expression required for custom frequency');
+    }
+  }
+
+  if (category && !['groceries', 'shopping', 'travel', 'restaurant', 'utilities', 'fun', 'settlement'].includes(category)) {
+    throw error(400, 'Invalid category');
+  }
+
+  // Validate personal + equal split method
+  if (splitMethod === 'personal_equal' && splits) {
+    const totalPersonal = splits.reduce((sum: number, split: any) => {
+      return sum + (parseFloat(split.personalAmount) || 0);
+    }, 0);
+    
+    if (totalPersonal > amount) {
+      throw error(400, 'Personal amounts cannot exceed total payment amount');
+    }
+  }
+
+  await dbConnect();
+  
+  try {
+    const recurringPaymentData = {
+      title,
+      description,
+      amount,
+      currency: 'CHF',
+      paidBy,
+      category: category || 'groceries',
+      splitMethod,
+      splits,
+      frequency,
+      cronExpression: frequency === 'custom' ? cronExpression : undefined,
+      startDate: startDate ? new Date(startDate) : new Date(),
+      endDate: endDate ? new Date(endDate) : undefined,
+      createdBy: auth.user.nickname,
+      isActive: true,
+      nextExecutionDate: new Date() // Will be calculated below
+    };
+
+    // Calculate the next execution date
+    recurringPaymentData.nextExecutionDate = calculateNextExecutionDate({
+      ...recurringPaymentData,
+      frequency,
+      cronExpression
+    } as any, recurringPaymentData.startDate);
+
+    const recurringPayment = await RecurringPayment.create(recurringPaymentData);
+
+    return json({ 
+      success: true, 
+      recurringPayment: recurringPayment._id 
+    });
+
+  } catch (e) {
+    console.error('Error creating recurring payment:', e);
+    throw error(500, 'Failed to create recurring payment');
+  } finally {
+    await dbDisconnect();
+  }
+};