Add missing Payment model and database connection utilities

- Created Payment model with mongoose schema for cospend functionality
- Added database connection utilities with proper connection caching
- Fixed build errors related to missing imports
- Build now succeeds and dev server starts correctly

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

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

src/lib/db/db.ts

@@ -0,0 +1,44 @@
+import mongoose from 'mongoose';
+
+const MONGODB_URI = process.env.MONGODB_URI || 'mongodb://localhost:27017/recipes';
+
+if (!MONGODB_URI) {
+  throw new Error('Please define the MONGODB_URI environment variable inside .env.local');
+}
+
+/**
+ * Global is used here to maintain a cached connection across hot reloads
+ * in development. This prevents connections growing exponentially
+ * during API Route usage.
+ */
+let cached = (global as any).mongoose;
+
+if (!cached) {
+  cached = (global as any).mongoose = { conn: null, promise: null };
+}
+
+export async function dbConnect() {
+  if (cached.conn) {
+    return cached.conn;
+  }
+
+  if (!cached.promise) {
+    const opts = {
+      bufferCommands: false,
+    };
+
+    cached.promise = mongoose.connect(MONGODB_URI, opts).then((mongoose) => {
+      return mongoose;
+    });
+  }
+  cached.conn = await cached.promise;
+  return cached.conn;
+}
+
+export async function dbDisconnect() {
+  if (cached.conn) {
+    await cached.conn.disconnect();
+    cached.conn = null;
+    cached.promise = null;
+  }
+}

src/lib/models/Payment.ts

@@ -0,0 +1,14 @@
+import mongoose from 'mongoose';
+
+const paymentSchema = new mongoose.Schema({
+  paid_by: { type: String, required: true },
+  total_amount: { type: Number, required: true },
+  for_self: { type: Number, default: 0 },
+  for_other: { type: Number, default: 0 },
+  currency: { type: String, default: 'CHF' },
+  description: String,
+  date: { type: Date, default: Date.now },
+  receipt_image: String
+});
+
+export const Payment = mongoose.models.Payment || mongoose.model('Payment', paymentSchema);