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

tests/test_crypto.py

@@ -0,0 +1,138 @@
+"""Tests for AES encryption (Phase 3.1)."""
+import pytest
+from app.core.crypto import encrypt_password, decrypt_password
+
+
+class TestEncryption:
+    def test_encrypt_decrypt_roundtrip(self):
+        """Encrypt then decrypt returns original."""
+        plain = "my_secret_password_123"
+        encrypted = encrypt_password(plain)
+        assert encrypted != plain
+        assert decrypt_password(encrypted) == plain
+
+    def test_encrypt_produces_different_ciphertext(self):
+        """Each encryption uses a random nonce, so ciphertext differs."""
+        plain = "same_password"
+        e1 = encrypt_password(plain)
+        e2 = encrypt_password(plain)
+        assert e1 != e2  # different nonces
+        assert decrypt_password(e1) == plain
+        assert decrypt_password(e2) == plain
+
+    def test_encrypt_empty_string(self):
+        """Can encrypt and decrypt empty string."""
+        encrypted = encrypt_password("")
+        assert decrypt_password(encrypted) == ""
+
+    def test_encrypt_special_characters(self):
+        """Handles special characters in passwords."""
+        plain = 'p@ssw0rd!#$%^&*()_+-=[]{}|;\':",./<>?'
+        encrypted = encrypt_password(plain)
+        assert decrypt_password(encrypted) == plain
+
+    def test_encrypt_unicode(self):
+        """Handles unicode characters."""
+        plain = "密码测试🔑"
+        encrypted = encrypt_password(plain)
+        assert decrypt_password(encrypted) == plain
+
+    def test_decrypt_invalid_data_returns_as_is(self):
+        """Invalid ciphertext returns original string (backward compat)."""
+        result = decrypt_password("not_valid_base64!!!")
+        assert result == "not_valid_base64!!!"
+
+
+class TestInstanceEncryption:
+    """Test that instance passwords are stored encrypted via API."""
+
+    @pytest.mark.asyncio
+    async def test_create_instance_encrypts_password(self, client, admin_token):
+        """POST /instances stores password as encrypted ciphertext."""
+        from tests.conftest import auth_header
+        resp = await client.post(
+            "/api/v1/instances",
+            json={
+                "name": "加密测试",
+                "db_type": "mysql",
+                "host": "10.0.0.1",
+                "port": 3306,
+                "username": "root",
+                "password": "plain_text_pw",
+                "database": "test",
+                "env": "development",
+            },
+            headers=auth_header(admin_token),
+        )
+        assert resp.status_code == 201
+        data = resp.json()
+        # Response should NOT contain password at all (InstanceResponse excludes it)
+        assert "password" not in data
+
+    @pytest.mark.asyncio
+    async def test_password_not_stored_as_plaintext(self, client, db, admin_token):
+        """Verify the database actually stores encrypted, not plain text."""
+        from tests.conftest import auth_header
+        from app.models.instance import Instance
+        from sqlalchemy import select
+
+        plain_pw = "super_secret_123"
+        resp = await client.post(
+            "/api/v1/instances",
+            json={
+                "name": "明文检查",
+                "db_type": "mysql",
+                "host": "10.0.0.2",
+                "port": 3306,
+                "username": "root",
+                "password": plain_pw,
+                "database": "test",
+                "env": "development",
+            },
+            headers=auth_header(admin_token),
+        )
+        assert resp.status_code == 201
+        instance_id = resp.json()["id"]
+
+        # Read raw from database
+        result = await db.execute(select(Instance).where(Instance.id == instance_id))
+        instance = result.scalar_one()
+        assert instance.password != plain_pw  # Not stored as plaintext
+        assert len(instance.password) > 20  # Encrypted is longer
+
+    @pytest.mark.asyncio
+    async def test_update_instance_re_encrypts_password(self, client, db, admin_token):
+        """PUT /instances/:id re-encrypts password when updated."""
+        from tests.conftest import auth_header
+        from app.models.instance import Instance
+        from sqlalchemy import select
+
+        # Create
+        resp = await client.post(
+            "/api/v1/instances",
+            json={
+                "name": "更新加密",
+                "db_type": "mysql",
+                "host": "10.0.0.3",
+                "port": 3306,
+                "username": "root",
+                "password": "old_password",
+                "database": "test",
+            },
+            headers=auth_header(admin_token),
+        )
+        iid = resp.json()["id"]
+
+        # Update password
+        resp = await client.put(
+            f"/api/v1/instances/{iid}",
+            json={"password": "new_password"},
+            headers=auth_header(admin_token),
+        )
+        assert resp.status_code == 200
+
+        # Verify encrypted
+        result = await db.execute(select(Instance).where(Instance.id == iid))
+        inst = result.scalar_one()
+        assert inst.password != "new_password"
+        assert inst.password != "old_password"