feat(payment): add complete payment system with multi-provider support

Add a full payment and subscription system supporting EasyPay (Alipay/WeChat),
Stripe, and direct Alipay/WeChat Pay providers with multi-instance load balancing.

backend/internal/payment/fee_test.go

@@ -0,0 +1,111 @@
+package payment
+
+import (
+	"testing"
+)
+
+func TestCalculatePayAmount(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		amount   float64
+		feeRate  float64
+		expected string
+	}{
+		{
+			name:     "zero fee rate returns same amount",
+			amount:   100.00,
+			feeRate:  0,
+			expected: "100.00",
+		},
+		{
+			name:     "negative fee rate returns same amount",
+			amount:   50.00,
+			feeRate:  -5,
+			expected: "50.00",
+		},
+		{
+			name:     "1 percent fee rate",
+			amount:   100.00,
+			feeRate:  1,
+			expected: "101.00",
+		},
+		{
+			name:     "5 percent fee on 200",
+			amount:   200.00,
+			feeRate:  5,
+			expected: "210.00",
+		},
+		{
+			name:     "fee rounds UP to 2 decimal places",
+			amount:   100.00,
+			feeRate:  3,
+			expected: "103.00",
+		},
+		{
+			name:     "fee rounds UP small remainder",
+			amount:   10.00,
+			feeRate:  3.33,
+			expected: "10.34", // 10 * 3.33 / 100 = 0.333 -> round up -> 0.34
+		},
+		{
+			name:     "very small amount",
+			amount:   0.01,
+			feeRate:  1,
+			expected: "0.02", // 0.01 * 1/100 = 0.0001 -> round up -> 0.01 -> total 0.02
+		},
+		{
+			name:     "large amount",
+			amount:   99999.99,
+			feeRate:  10,
+			expected: "109999.99", // 99999.99 * 10/100 = 9999.999 -> round up -> 10000.00 -> total 109999.99
+		},
+		{
+			name:     "100 percent fee rate doubles amount",
+			amount:   50.00,
+			feeRate:  100,
+			expected: "100.00",
+		},
+		{
+			name:     "precision 0.01 fee difference",
+			amount:   100.00,
+			feeRate:  1.01,
+			expected: "101.01", // 100 * 1.01/100 = 1.01
+		},
+		{
+			name:     "precision 0.02 fee",
+			amount:   100.00,
+			feeRate:  1.02,
+			expected: "101.02",
+		},
+		{
+			name:     "zero amount with positive fee",
+			amount:   0,
+			feeRate:  5,
+			expected: "0.00",
+		},
+		{
+			name:     "fractional amount no fee",
+			amount:   19.99,
+			feeRate:  0,
+			expected: "19.99",
+		},
+		{
+			name:     "fractional fee that causes rounding up",
+			amount:   33.33,
+			feeRate:  7.77,
+			expected: "35.92", // 33.33 * 7.77 / 100 = 2.589741 -> round up -> 2.59 -> total 35.92
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got := CalculatePayAmount(tt.amount, tt.feeRate)
+			if got != tt.expected {
+				t.Fatalf("CalculatePayAmount(%v, %v) = %q, want %q", tt.amount, tt.feeRate, got, tt.expected)
+			}
+		})
+	}
+}