Fix DataJoint syntax in HashRegistry definition

- Use '= CURRENT_TIMESTAMP : datetime' syntax (not SQL DEFAULT)
- Use uint64 core type instead of 'bigint unsigned' native type

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

Author: claude

docs/src/design/tables/storage-types-spec.md

@@ -611,8 +611,8 @@ class HashRegistry:
     hash_id : char(64)          # SHA256 hex
     ---
     store        : varchar(64)       # Store name
-    size         : bigint unsigned   # Size in bytes
-    created      : timestamp DEFAULT CURRENT_TIMESTAMP
+    size         : uint64            # Size in bytes
+    created = CURRENT_TIMESTAMP : datetime
     """
 ```