Move EXTERNAL_TABLE_ROOT to external.py (deprecated)

External tables are deprecated in favor of the new storage type system.
Move the constant to external.py where it's used, keeping declare.py clean.

Co-authored-by: dimitri-yatsenko <dimitri@datajoint.com>

Author: claude

src/datajoint/external.py

@@ -5,15 +5,18 @@
 
 from tqdm import tqdm
 
-from .declare import EXTERNAL_TABLE_ROOT
 from .errors import DataJointError, MissingExternalFile
 from .hash import uuid_from_buffer, uuid_from_file
 from .heading import Heading
 from .settings import config
 from .storage import StorageBackend
 from .table import FreeTable, Table
+
 from .utils import safe_write
 
+# External table name root (deprecated - external tables are being phased out)
+EXTERNAL_TABLE_ROOT = "~external"
+
 logger = logging.getLogger(__name__.split(".")[0])
 
 CACHE_SUBFOLDING = (