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/models/RecurringPayment.ts

@@ -0,0 +1,141 @@
+import mongoose from 'mongoose';
+
+export interface IRecurringPayment {
+  _id?: string;
+  title: string;
+  description?: string;
+  amount: number;
+  currency: string;
+  paidBy: string; // username/nickname of the person who paid
+  category: 'groceries' | 'shopping' | 'travel' | 'restaurant' | 'utilities' | 'fun' | 'settlement';
+  splitMethod: 'equal' | 'full' | 'proportional' | 'personal_equal';
+  splits: Array<{
+    username: string;
+    amount?: number;
+    proportion?: number;
+    personalAmount?: number;
+  }>;
+  frequency: 'daily' | 'weekly' | 'monthly' | 'custom';
+  cronExpression?: string; // For custom frequencies using cron syntax
+  isActive: boolean;
+  nextExecutionDate: Date;
+  lastExecutionDate?: Date;
+  startDate: Date;
+  endDate?: Date; // Optional end date for the recurring payments
+  createdBy: string;
+  createdAt?: Date;
+  updatedAt?: Date;
+}
+
+const RecurringPaymentSchema = new mongoose.Schema(
+  {
+    title: {
+      type: String,
+      required: true,
+      trim: true
+    },
+    description: {
+      type: String,
+      trim: true
+    },
+    amount: {
+      type: Number,
+      required: true,
+      min: 0
+    },
+    currency: {
+      type: String,
+      required: true,
+      default: 'CHF',
+      enum: ['CHF']
+    },
+    paidBy: {
+      type: String,
+      required: true,
+      trim: true
+    },
+    category: {
+      type: String,
+      required: true,
+      enum: ['groceries', 'shopping', 'travel', 'restaurant', 'utilities', 'fun', 'settlement'],
+      default: 'groceries'
+    },
+    splitMethod: {
+      type: String,
+      required: true,
+      enum: ['equal', 'full', 'proportional', 'personal_equal'],
+      default: 'equal'
+    },
+    splits: [{
+      username: {
+        type: String,
+        required: true,
+        trim: true
+      },
+      amount: {
+        type: Number
+      },
+      proportion: {
+        type: Number,
+        min: 0,
+        max: 1
+      },
+      personalAmount: {
+        type: Number,
+        min: 0
+      }
+    }],
+    frequency: {
+      type: String,
+      required: true,
+      enum: ['daily', 'weekly', 'monthly', 'custom']
+    },
+    cronExpression: {
+      type: String,
+      validate: {
+        validator: function(value: string) {
+          // Only validate if frequency is custom
+          if (this.frequency === 'custom') {
+            return value != null && value.trim().length > 0;
+          }
+          return true;
+        },
+        message: 'Cron expression is required when frequency is custom'
+      }
+    },
+    isActive: {
+      type: Boolean,
+      default: true
+    },
+    nextExecutionDate: {
+      type: Date,
+      required: true
+    },
+    lastExecutionDate: {
+      type: Date
+    },
+    startDate: {
+      type: Date,
+      required: true,
+      default: Date.now
+    },
+    endDate: {
+      type: Date
+    },
+    createdBy: {
+      type: String,
+      required: true,
+      trim: true
+    }
+  },
+  {
+    timestamps: true,
+    toJSON: { virtuals: true },
+    toObject: { virtuals: true }
+  }
+);
+
+// Index for efficiently finding payments that need to be executed
+RecurringPaymentSchema.index({ nextExecutionDate: 1, isActive: 1 });
+
+export const RecurringPayment = mongoose.model<IRecurringPayment>("RecurringPayment", RecurringPaymentSchema);