feat: Go后端LDAP真实对接 — AES-256-GCM密码加解密 + 登录LDAP fallback

- 新增 crypto.go: AES-256-GCM 加解密，与 Python backend 兼容
- auth.go: Login 添加 LDAP fallback（本地优先→LDAP绑定→自动创建本地用户）
- ldap.go: 实现真实 LDAP 连接/用户搜索/密码验证/OU列表/用户列表
- 所有 LDAP 操作解密 admin_password 后使用
- 新增/更新密码存储时加密
- router.go: 补充 delete-user 路由
- 添加 go-ldap/v3 依赖

backend/internal/handler/crypto.go

@@ -0,0 +1,83 @@
+package handler
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"os"
+)
+
+// getSecretKey returns the AES key derived from SECRET_KEY env var.
+// Mirrors Python's crypto._get_key(): first 32 bytes of utf-8 encoded key, zero-padded.
+func getSecretKey() []byte {
+	keyStr := os.Getenv("SECRET_KEY")
+	if keyStr == "" {
+		keyStr = "dev-secret-key-change-in-production"
+	}
+	raw := []byte(keyStr)
+	if len(raw) >= 32 {
+		return raw[:32]
+	}
+	padded := make([]byte, 32)
+	copy(padded, raw)
+	return padded
+}
+
+// decryptPassword decrypts a base64-encoded AES-256-GCM ciphertext.
+// Mirrors Python's crypto.decrypt_password().
+// If the value is not valid encrypted data (e.g., plaintext), returns it as-is.
+func decryptPassword(encrypted string) (string, error) {
+	raw, err := base64.StdEncoding.DecodeString(encrypted)
+	if err != nil {
+		return encrypted, nil // not valid base64 — plaintext
+	}
+
+	nonceSize := 12
+	if len(raw) <= nonceSize {
+		return encrypted, nil
+	}
+
+	nonce := raw[:nonceSize]
+	ct := raw[nonceSize:]
+	key := getSecretKey()
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return "", err
+	}
+
+	aesgcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+
+	plaintext, err := aesgcm.Open(nil, nonce, ct, nil)
+	if err != nil {
+		return encrypted, nil // decryption failed — return as-is
+	}
+
+	return string(plaintext), nil
+}
+
+// encryptPassword encrypts a plaintext password using AES-256-GCM.
+// Returns base64(nonce + ciphertext), same format as Python's encrypt_password.
+func encryptPassword(plaintext string) (string, error) {
+	key := getSecretKey()
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return "", err
+	}
+	aesgcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+
+	nonce := make([]byte, 12)
+	if _, err := rand.Read(nonce); err != nil {
+		return "", err
+	}
+
+	ct := aesgcm.Seal(nil, nonce, []byte(plaintext), nil)
+	return base64.StdEncoding.EncodeToString(append(nonce, ct...)), nil
+}