Add retry logic (3 attempts with backoff) to CZDS zone downloads

Large zone files (100-200MB) were failing due to connection interruptions.
Now retries up to 3 times with 30s/60s/90s backoff between attempts.

backend/scripts/sync_all_zones.py

@@ -92,8 +92,11 @@ async def get_db_session():
     return async_session()
 
 
-def download_czds_zone(tld: str) -> Optional[Path]:
+def download_czds_zone(tld: str, max_retries: int = 3) -> Optional[Path]:
-    """Download a single CZDS zone file using pyCZDS"""
+    """Download a single CZDS zone file using pyCZDS with retry logic"""
+    import time
+    
+    for attempt in range(max_retries):
         try:
             from pyczds.client import CZDSClient
             
@@ -129,7 +132,7 @@ def download_czds_zone(tld: str) -> Optional[Path]:
                 logger.warning(f"No access to .{tld} zone file")
                 return None
             
-        logger.info(f"Downloading .{tld} from CZDS...")
+            logger.info(f"Downloading .{tld} from CZDS... (attempt {attempt + 1}/{max_retries})")
             result = client.get_zonefile(target_url, download_dir=str(CZDS_DIR))
             
             # Find the downloaded file
@@ -144,7 +147,15 @@ def download_czds_zone(tld: str) -> Optional[Path]:
             return None
             
         except Exception as e:
-        logger.error(f"CZDS download failed for .{tld}: {e}")
+            logger.warning(f"CZDS download attempt {attempt + 1} failed for .{tld}: {e}")
+            if attempt < max_retries - 1:
+                wait_time = (attempt + 1) * 30  # 30s, 60s, 90s backoff
+                logger.info(f"Retrying in {wait_time}s...")
+                time.sleep(wait_time)
+            else:
+                logger.error(f"CZDS download failed for .{tld} after {max_retries} attempts")
+                return None
+    
     return None