refactor: deduplicate ItemInfo, improve error handling, fix pre-existing bugs

- Move ItemInfo to services/types.rs for sharing between client and server
- Replace .expect() in compression_service with proper error handling
- Add CoreError::PayloadTooLarge variant for semantic error handling
- Export CoreError from lib.rs for library users
- Unify get_item_meta_name/value to take &str instead of String
- Extract item_path() helper in ItemService to reduce duplication
- Add warning logs for silent errors in list.rs
- Fix pre-existing borrow errors: tx moved in export handler,
  item_with_meta partial move in TryFrom implementation
- Fix unused data_dir variables in server code

src/modes/common.rs

@@ -61,6 +61,9 @@ lazy_static! {
     static ref KEEP_META_RE: Regex = Regex::new(r"^KEEP_META_(.+)$").unwrap();
 }
 
+pub const IMPORT_FORMAT_ERROR: &str =
+    "Unsupported import format: {} (expected .keep.tar or .meta.yml)";
+
 pub fn get_meta_from_env() -> HashMap<String, String> {
     debug!("COMMON: Getting meta from KEEP_META_*");
     let mut meta_env: HashMap<String, String> = HashMap::new();