security: auth hatasi 401 yerine 404 don

Ne yaptik:
- verify_api_key: HTTPException 401 -> 404, detail 'Not Found'

Neden yaptik:
- API key'siz erisimde JSON hata mesaji gorunuyordu
- 404 ile endpoint yokmus gibi gozukur, domain ifsa olmaz

backend/main.py

@@ -65,10 +65,7 @@ async def verify_api_key(request: Request, api_key: Optional[str] = Security(API
         # API_KEY env var not set — authentication disabled
         return
     if not api_key or api_key != API_KEY:
-        raise HTTPException(
-            status_code=401,
-            detail="Invalid or missing API key"
-        )
+        raise HTTPException(status_code=404, detail="Not Found")
 
 # GS1 Barcode Prefix to Country Mapping (EAN-13 / EAN-8)
 # Source: https://www.gs1.org/standards/id-keys/company-prefix