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/routers/import_kimai.py

@@ -19,6 +19,19 @@ router = APIRouter(prefix="/import", tags=["import"])
 
 _allowed_roles = [UserRole.HR, UserRole.COMPANY_ADMIN, UserRole.SUPER_ADMIN]
 
+_MAX_UPLOAD_BYTES = 10 * 1024 * 1024  # 10 MB
+
+
+async def _read_upload(file: UploadFile) -> bytes:
+    """Liest eine UploadFile mit Größenbegrenzung (max 10 MB)."""
+    content = await file.read(_MAX_UPLOAD_BYTES + 1)
+    if len(content) > _MAX_UPLOAD_BYTES:
+        raise HTTPException(
+            status_code=413,
+            detail=f"Datei zu groß. Maximale Upload-Größe: {_MAX_UPLOAD_BYTES // (1024 * 1024)} MB.",
+        )
+    return content
+
 
 class ImportPreviewResponse(BaseModel):
     preview: list[ImportPreviewEntry]
@@ -48,7 +61,7 @@ async def kimai_preview(
     except ValueError:
         raise HTTPException(status_code=400, detail="Ungültige user_id")
 
-    content = await file.read()
+    content = await _read_upload(file)
     result: ImportResult = await preview_kimai_import(content, target_id, db)
 
     time_count = sum(1 for p in result.preview if p.kind == "time" and not p.skipped)
@@ -76,7 +89,7 @@ async def kimai_run(
     except ValueError:
         raise HTTPException(status_code=400, detail="Ungültige user_id")
 
-    content = await file.read()
+    content = await _read_upload(file)
     result: ImportResult = await run_kimai_import(content, target_id, current_user.id, db)
 
     return ImportRunResponse(