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/crypto.go

@@ -0,0 +1,98 @@
+package payment
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"fmt"
+	"io"
+	"strings"
+)
+
+// Encrypt encrypts plaintext using AES-256-GCM with the given 32-byte key.
+// The output format is "iv:authTag:ciphertext" where each component is base64-encoded,
+// matching the Node.js crypto.ts format for cross-compatibility.
+func Encrypt(plaintext string, key []byte) (string, error) {
+	if len(key) != 32 {
+		return "", fmt.Errorf("encryption key must be 32 bytes, got %d", len(key))
+	}
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return "", fmt.Errorf("create AES cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", fmt.Errorf("create GCM: %w", err)
+	}
+
+	nonce := make([]byte, gcm.NonceSize()) // 12 bytes for GCM
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", fmt.Errorf("generate nonce: %w", err)
+	}
+
+	// Seal appends the ciphertext + auth tag
+	sealed := gcm.Seal(nil, nonce, []byte(plaintext), nil)
+
+	// Split sealed into ciphertext and auth tag (last 16 bytes)
+	tagSize := gcm.Overhead()
+	ciphertext := sealed[:len(sealed)-tagSize]
+	authTag := sealed[len(sealed)-tagSize:]
+
+	// Format: iv:authTag:ciphertext (all base64)
+	return fmt.Sprintf("%s:%s:%s",
+		base64.StdEncoding.EncodeToString(nonce),
+		base64.StdEncoding.EncodeToString(authTag),
+		base64.StdEncoding.EncodeToString(ciphertext),
+	), nil
+}
+
+// Decrypt decrypts a ciphertext string produced by Encrypt.
+// The input format is "iv:authTag:ciphertext" where each component is base64-encoded.
+func Decrypt(ciphertext string, key []byte) (string, error) {
+	if len(key) != 32 {
+		return "", fmt.Errorf("encryption key must be 32 bytes, got %d", len(key))
+	}
+
+	parts := strings.SplitN(ciphertext, ":", 3)
+	if len(parts) != 3 {
+		return "", fmt.Errorf("invalid ciphertext format: expected iv:authTag:ciphertext")
+	}
+
+	nonce, err := base64.StdEncoding.DecodeString(parts[0])
+	if err != nil {
+		return "", fmt.Errorf("decode IV: %w", err)
+	}
+
+	authTag, err := base64.StdEncoding.DecodeString(parts[1])
+	if err != nil {
+		return "", fmt.Errorf("decode auth tag: %w", err)
+	}
+
+	encrypted, err := base64.StdEncoding.DecodeString(parts[2])
+	if err != nil {
+		return "", fmt.Errorf("decode ciphertext: %w", err)
+	}
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return "", fmt.Errorf("create AES cipher: %w", err)
+	}
+
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", fmt.Errorf("create GCM: %w", err)
+	}
+
+	// Reconstruct the sealed data: ciphertext + authTag
+	sealed := append(encrypted, authTag...)
+
+	plaintext, err := gcm.Open(nil, nonce, sealed, nil)
+	if err != nil {
+		return "", fmt.Errorf("decrypt: %w", err)
+	}
+
+	return string(plaintext), nil
+}