Fix: Retry when downloaded CZDS file not found

Previously, if download succeeded but file wasn't found, function
returned None immediately. Now raises FileNotFoundError to trigger
the retry logic properly.

backend/scripts/sync_all_zones.py

@@ -140,11 +140,12 @@ def download_czds_zone(tld: str, max_retries: int = 3) -> Optional[Path]:
             if gz_file.exists():
                 return gz_file
             
-            # Try alternative naming
+            # Try alternative naming (pyCZDS sometimes uses different names)
             for f in CZDS_DIR.glob(f"*{tld}*.gz"):
                 return f
-                
-            return None
+            
+            # File not found after download - raise exception to trigger retry
+            raise FileNotFoundError(f"Downloaded file not found for .{tld} in {CZDS_DIR}")
             
         except Exception as e:
             logger.warning(f"CZDS download attempt {attempt + 1} failed for .{tld}: {e}")