feat(payments): add Payment model for Phase 1 receipt data capture

Introduces the payments collection with fields Phase 2 will need to
generate official donation receipts: amount, paymentDate, paymentType
(monthly|annual), helcimTransactionId (unique), and receiptIssued /
receiptId placeholders Phase 2 will populate. Schema-only; no routes
or UI in this commit.

server/models/payment.js

@@ -0,0 +1,28 @@
+import mongoose from 'mongoose'
+
+const paymentSchema = new mongoose.Schema(
+  {
+    memberId: { type: mongoose.Schema.Types.ObjectId, ref: 'Member', required: true, index: true },
+    helcimTransactionId: { type: String, required: true, unique: true },
+    helcimCustomerId: { type: String, index: true },
+    helcimSubscriptionId: { type: String, index: true, default: null },
+
+    amount: { type: Number, required: true, min: 0 },
+    currency: { type: String, default: 'CAD' },
+    paymentDate: { type: Date, required: true },
+
+    paymentType: { type: String, enum: ['monthly', 'annual'], required: true },
+    status: { type: String, enum: ['success', 'failed', 'refunded'], required: true, default: 'success' },
+    failureReason: { type: String, default: null },
+
+    receiptIssued: { type: Boolean, default: false },
+    receiptId: { type: String, default: null },
+
+    rawHelcim: { type: mongoose.Schema.Types.Mixed, default: null },
+  },
+  { timestamps: true, collection: 'payments' }
+)
+
+paymentSchema.index({ memberId: 1, paymentDate: -1 })
+
+export default mongoose.models.Payment || mongoose.model('Payment', paymentSchema)