security: 9 Findings aus Security-Audit behoben (CRITICAL + HIGH + MEDIUM)

CRITICAL:
- C-1: LDAP tls_verify Default False → True (MITM-Schutz)
- C-2: TOTP-Secret Fernet-verschlüsselt in DB (statt Plaintext)
  - core/crypto.py: encrypt_value() / decrypt_value() helper
  - Migration 0026: totp_secret VARCHAR(64→500), ldap tls_verify default=true
  - _totp_plain() helper mit Legacy-Fallback für bestehende Werte

HIGH:
- H-1: Kiosk Nonce-Cache asyncio.Lock (Race Condition behoben)
- H-2: File-Upload-Limit 10 MB (import_kimai.py + users.py CSV-Import)
- H-3: CORS allow_methods/allow_headers explizit eingeschränkt (war *)
- H-4: TrustedHostMiddleware aktiviert wenn ALLOWED_HOSTS gesetzt

MEDIUM:
- M-1: IP-Logging nutzt X-Forwarded-For hinter nginx-Proxy
- M-4: Audit-Log für password_changed, totp_enabled, totp_disabled
- M-5: CalDAV verify_ssl in Production erzwungen (_effective_verify_ssl)

152/152 Tests grün

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

backend/app/models/ldap_config.py

@@ -27,7 +27,7 @@ class LdapConfig(Base):
     port: Mapped[int] = mapped_column(Integer, default=389)
     use_ssl: Mapped[bool] = mapped_column(Boolean, default=False)
     use_tls: Mapped[bool] = mapped_column(Boolean, default=False)
-    tls_verify: Mapped[bool] = mapped_column(Boolean, default=False)
+    tls_verify: Mapped[bool] = mapped_column(Boolean, default=True)
 
     # Bind credentials
     bind_dn: Mapped[str] = mapped_column(Text, nullable=False)