fix(security): behebe F-01/F-02/W-03/W-04 aus Security-Audit + PROJ-24 TOTP 2FA

F-01: err.Error() wird nicht mehr an HTTP-Clients gesendet.
      Stattdessen generische Fehlermeldungen + Server-Log.
      Betrifft: handleCreateUser, handleUpdateUser, handleDeleteUser,
                handleSyncNow, handleSecurityConfig, handleUpload.

F-02: Login-Audit-Log enthält keinen rohen err.Error() mehr.
      Neue classifyLoginError() Funktion: invalid_password / ldap_error /
      account_disabled / unknown — schützt vor LDAP-Info-Leak via Audit-API.

W-03: remoteIP() trimmt jetzt Leerzeichen aus X-Forwarded-For.
      Vollständige Lösung erfordert Proxy-Konfiguration (W-03 bleibt WARN).

W-04: Attachment-Dateiname wird durch sanitizeFilename() bereinigt.
      Nur [a-zA-Z0-9._- ] erlaubt — verhindert Header-Injection.

PROJ-24: TOTP 2FA vollständig implementiert:
      - internal/auth/totp.go: GenerateSecret, ValidateTOTP, QRCodeSVG
      - internal/api/totp_handlers.go: Setup, Login-Step2, Admin-Reset
      - internal/userstore: SetTOTPSecret, EnableTOTP, DisableTOTP, ResetTOTP
      - Login-Flow: totp_pending JWT → /api/auth/totp → vollwertiger JWT
      - AES-256-GCM verschlüsseltes Secret in users.totp_secret

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/auth/auth.go

@@ -2,10 +2,13 @@ package auth
 
 import (
 	"context"
+	"crypto/aes"
+	"crypto/cipher"
 	"crypto/rand"
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"io"
 	"strings"
 	"time"
 
@@ -37,17 +40,21 @@ type Manager struct {
 	store           *userstore.Store
 	ldapStore       *ldapcfg.Store
 	jwtSecret       []byte
+	aesKey          []byte                 // PROJ-24: AES-256-GCM key for TOTP secret encryption
 	tenantLdapStore *ldapcfg.TenantStore   // PROJ-23: per-tenant LDAP config
 	tenantLookup    TenantDomainLookup     // PROJ-23: domain -> tenant_id resolution
 }
 
 // New creates a new auth Manager.
 // ldapStore may be nil; in that case LDAP fallback is disabled.
-func New(store *userstore.Store, ldapStore *ldapcfg.Store, jwtSecret string) *Manager {
+// aesKey is the hex-encoded AES-256 key for encrypting TOTP secrets.
+func New(store *userstore.Store, ldapStore *ldapcfg.Store, jwtSecret string, aesKey string) *Manager {
+	aesKeyBytes, _ := hex.DecodeString(aesKey)
 	return &Manager{
 		store:     store,
 		ldapStore: ldapStore,
 		jwtSecret: []byte(jwtSecret),
+		aesKey:    aesKeyBytes,
 	}
 }
 
@@ -61,11 +68,18 @@ func (m *Manager) SetTenantLDAP(tenantLdapStore *ldapcfg.TenantStore, tenantLook
 // Login verifies credentials and returns a signed JWT token.
 // It first attempts a local password check. If that fails and LDAP is
 // configured and enabled, it falls back to LDAP authentication.
-func (m *Manager) Login(username, password string) (string, *userstore.User, error) {
+// If the user has TOTP enabled, totpRequired is true and the token is a
+// short-lived pending token that can only be used with ValidateTOTPLogin.
+func (m *Manager) Login(username, password string) (token string, user *userstore.User, totpRequired bool, err error) {
 	// 1. Try local authentication first.
-	user, err := m.store.VerifyPassword(username, password)
+	user, err = m.store.VerifyPassword(username, password)
 	if err == nil {
-		return m.issueToken(user)
+		if user.TOTPEnabled {
+			t, e := m.issuePendingTOTPToken(user)
+			return t, user, true, e
+		}
+		t, u, e := m.issueToken(user)
+		return t, u, false, e
 	}
 
 	// 2. PROJ-23: Per-tenant LDAP — checked first so tenant config takes priority
@@ -107,7 +121,12 @@ func (m *Manager) Login(username, password string) (string, *userstore.User, err
 						}
 						ldapUser, upsertErr := m.store.UpsertLDAPUser(username, email, role, tenantID)
 						if upsertErr == nil {
-							return m.issueToken(ldapUser)
+							if ldapUser.TOTPEnabled {
+								t, e := m.issuePendingTOTPToken(ldapUser)
+								return t, ldapUser, true, e
+							}
+							t, u, e := m.issueToken(ldapUser)
+							return t, u, false, e
 						}
 					}
 				}
@@ -148,13 +167,18 @@ func (m *Manager) Login(username, password string) (string, *userstore.User, err
 				}
 				ldapUser, upsertErr := m.store.UpsertLDAPUser(username, email, role, nil)
 				if upsertErr == nil {
-					return m.issueToken(ldapUser)
+					if ldapUser.TOTPEnabled {
+						t, e := m.issuePendingTOTPToken(ldapUser)
+						return t, ldapUser, true, e
+					}
+					t, u, e := m.issueToken(ldapUser)
+					return t, u, false, e
 				}
 			}
 		}
 	}
 
-	return "", nil, fmt.Errorf("auth: login: invalid credentials")
+	return "", nil, false, fmt.Errorf("auth: login: invalid credentials")
 }
 
 // issueToken signs a JWT for the given user and returns the token string.
@@ -186,6 +210,99 @@ func (m *Manager) issueToken(user *userstore.User) (string, *userstore.User, err
 	return signed, user, nil
 }
 
+// issuePendingTOTPToken issues a short-lived JWT (5 min) that signals TOTP is required.
+// This token MUST NOT be accepted as a full auth token — only for /api/auth/totp.
+func (m *Manager) issuePendingTOTPToken(user *userstore.User) (string, error) {
+	jti := generateJTI()
+	now := time.Now()
+
+	claims := jwt.MapClaims{
+		"sub":          user.Username,
+		"uid":          user.ID,
+		"jti":          jti,
+		"iat":          now.Unix(),
+		"exp":          now.Add(5 * time.Minute).Unix(),
+		"totp_pending": true,
+	}
+
+	token := jwt.NewWithClaims(jwt.SigningMethodHS256, claims)
+	signed, err := token.SignedString(m.jwtSecret)
+	if err != nil {
+		return "", fmt.Errorf("auth: sign pending totp token: %w", err)
+	}
+	return signed, nil
+}
+
+// ValidateTOTPLogin validates a pending TOTP token and TOTP code, then issues a full JWT.
+func (m *Manager) ValidateTOTPLogin(pendingToken, code string) (string, *userstore.User, error) {
+	// Parse the pending token
+	token, err := jwt.Parse(pendingToken, func(t *jwt.Token) (interface{}, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("auth: unexpected signing method: %v", t.Header["alg"])
+		}
+		return m.jwtSecret, nil
+	})
+	if err != nil {
+		return "", nil, fmt.Errorf("auth: invalid pending token: %w", err)
+	}
+	if !token.Valid {
+		return "", nil, errors.New("auth: pending token not valid")
+	}
+
+	claims, ok := token.Claims.(jwt.MapClaims)
+	if !ok {
+		return "", nil, errors.New("auth: bad claims")
+	}
+
+	// Verify this is actually a pending TOTP token
+	pending, _ := claims["totp_pending"].(bool)
+	if !pending {
+		return "", nil, errors.New("auth: not a pending TOTP token")
+	}
+
+	// Extract user ID
+	var userID int64
+	switch v := claims["uid"].(type) {
+	case float64:
+		userID = int64(v)
+	case int64:
+		userID = v
+	default:
+		return "", nil, errors.New("auth: missing uid in pending token")
+	}
+
+	// Load user from DB
+	user, err := m.store.GetByID(userID)
+	if err != nil {
+		return "", nil, fmt.Errorf("auth: user lookup: %w", err)
+	}
+	if !user.TOTPEnabled {
+		return "", nil, errors.New("auth: TOTP not enabled for this user")
+	}
+
+	// Load and decrypt TOTP secret
+	encSecret, enabled, err := m.store.GetTOTPSecret(context.Background(), userID)
+	if err != nil {
+		return "", nil, fmt.Errorf("auth: get totp secret: %w", err)
+	}
+	if !enabled || len(encSecret) == 0 {
+		return "", nil, errors.New("auth: TOTP not configured")
+	}
+
+	plainSecret, err := m.decryptAES(encSecret)
+	if err != nil {
+		return "", nil, fmt.Errorf("auth: decrypt totp secret: %w", err)
+	}
+
+	// Validate the TOTP code
+	if !ValidateTOTP(string(plainSecret), code) {
+		return "", nil, errors.New("auth: invalid TOTP code")
+	}
+
+	// Issue a full auth token
+	return m.issueToken(user)
+}
+
 // ValidateToken parses and validates the token, checking the blacklist.
 func (m *Manager) ValidateToken(tokenStr string) (*Session, error) {
 	token, err := jwt.Parse(tokenStr, func(t *jwt.Token) (interface{}, error) {
@@ -206,6 +323,11 @@ func (m *Manager) ValidateToken(tokenStr string) (*Session, error) {
 		return nil, errors.New("auth: bad claims")
 	}
 
+	// PROJ-24: Reject pending TOTP tokens — they must not be used as full auth tokens.
+	if pending, _ := claims["totp_pending"].(bool); pending {
+		return nil, errors.New("auth: pending TOTP token cannot be used for authentication")
+	}
+
 	jti, _ := claims["jti"].(string)
 	blacklisted, err := m.store.IsBlacklisted(jti)
 	if err != nil {
@@ -361,3 +483,64 @@ func generateJTI() string {
 	}
 	return hex.EncodeToString(b)
 }
+
+// ── PROJ-24: AES-256-GCM encryption helpers for TOTP secrets ─────────────
+
+// EncryptAES encrypts plaintext using AES-256-GCM with the manager's AES key.
+// The 12-byte nonce is prepended to the ciphertext.
+func (m *Manager) EncryptAES(plaintext []byte) ([]byte, error) {
+	return encryptAESGCM(m.aesKey, plaintext)
+}
+
+// decryptAES decrypts data encrypted with EncryptAES.
+func (m *Manager) decryptAES(data []byte) ([]byte, error) {
+	return decryptAESGCM(m.aesKey, data)
+}
+
+// DecryptAESForHandler exposes AES decryption for use in API handlers (e.g., TOTP setup confirmation).
+func (m *Manager) DecryptAESForHandler(data []byte) ([]byte, error) {
+	return decryptAESGCM(m.aesKey, data)
+}
+
+func encryptAESGCM(key, plaintext []byte) ([]byte, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, fmt.Errorf("auth: aes cipher: %w", err)
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, fmt.Errorf("auth: gcm: %w", err)
+	}
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, fmt.Errorf("auth: random nonce: %w", err)
+	}
+	ciphertext := gcm.Seal(nonce, nonce, plaintext, nil)
+	return ciphertext, nil
+}
+
+func decryptAESGCM(key, data []byte) ([]byte, error) {
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return nil, fmt.Errorf("auth: aes cipher: %w", err)
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, fmt.Errorf("auth: gcm: %w", err)
+	}
+	nonceSize := gcm.NonceSize()
+	if len(data) < nonceSize {
+		return nil, fmt.Errorf("auth: ciphertext too short")
+	}
+	nonce, ciphertext := data[:nonceSize], data[nonceSize:]
+	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
+	if err != nil {
+		return nil, fmt.Errorf("auth: decrypt: %w", err)
+	}
+	return plaintext, nil
+}
+
+// GetUserStore returns the underlying user store (used by TOTP handlers).
+func (m *Manager) GetUserStore() *userstore.Store {
+	return m.store
+}