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

@@ -92,60 +92,71 @@ async def get_db_session():
     return async_session()
 
 
-def download_czds_zone(tld: str) -> Optional[Path]:
-    """Download a single CZDS zone file using pyCZDS"""
-    try:
-        from pyczds.client import CZDSClient
+def download_czds_zone(tld: str, max_retries: int = 3) -> Optional[Path]:
+    """Download a single CZDS zone file using pyCZDS with retry logic"""
+    import time
     
-        # Read credentials from .env
-        env_file = Path(__file__).parent.parent / ".env"
-        if not env_file.exists():
-            env_file = Path("/home/user/pounce/backend/.env")
+    for attempt in range(max_retries):
+        try:
+            from pyczds.client import CZDSClient
             
-        env_content = env_file.read_text()
-        username = password = None
+            # Read credentials from .env
+            env_file = Path(__file__).parent.parent / ".env"
+            if not env_file.exists():
+                env_file = Path("/home/user/pounce/backend/.env")
             
-        for line in env_content.splitlines():
-            if line.startswith("CZDS_USERNAME="):
-                username = line.split("=", 1)[1].strip()
-            elif line.startswith("CZDS_PASSWORD="):
-                password = line.split("=", 1)[1].strip()
+            env_content = env_file.read_text()
+            username = password = None
+            
+            for line in env_content.splitlines():
+                if line.startswith("CZDS_USERNAME="):
+                    username = line.split("=", 1)[1].strip()
+                elif line.startswith("CZDS_PASSWORD="):
+                    password = line.split("=", 1)[1].strip()
+            
+            if not username or not password:
+                logger.error(f"CZDS credentials not found in .env")
+                return None
+            
+            client = CZDSClient(username, password)
+            urls = client.get_zonefiles_list()
+            
+            # Find URL for this TLD
+            target_url = None
+            for url in urls:
+                if f"{tld}.zone" in url or f"/{tld}." in url:
+                    target_url = url
+                    break
+            
+            if not target_url:
+                logger.warning(f"No access to .{tld} zone file")
+                return None
+            
+            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
+            gz_file = CZDS_DIR / f"{tld}.txt.gz"
+            if gz_file.exists():
+                return gz_file
+            
+            # Try alternative naming
+            for f in CZDS_DIR.glob(f"*{tld}*.gz"):
+                return f
                 
-        if not username or not password:
-            logger.error(f"CZDS credentials not found in .env")
             return None
             
-        client = CZDSClient(username, password)
-        urls = client.get_zonefiles_list()
+        except Exception as 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
     
-        # Find URL for this TLD
-        target_url = None
-        for url in urls:
-            if f"{tld}.zone" in url or f"/{tld}." in url:
-                target_url = url
-                break
-        
-        if not target_url:
-            logger.warning(f"No access to .{tld} zone file")
-            return None
-        
-        logger.info(f"Downloading .{tld} from CZDS...")
-        result = client.get_zonefile(target_url, download_dir=str(CZDS_DIR))
-        
-        # Find the downloaded file
-        gz_file = CZDS_DIR / f"{tld}.txt.gz"
-        if gz_file.exists():
-            return gz_file
-        
-        # Try alternative naming
-        for f in CZDS_DIR.glob(f"*{tld}*.gz"):
-            return f
-            
-        return None
-        
-    except Exception as e:
-        logger.error(f"CZDS download failed for .{tld}: {e}")
-        return None
+    return None
 
 
 def download_switch_zone(tld: str) -> Optional[Path]: