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/services/mod.rs

@@ -13,5 +13,5 @@ pub use filter_service::{FilterService, register_filter_plugin};
 pub use item_service::ItemService;
 pub use meta_service::MetaService;
 pub use status_service::StatusService;
-pub use types::{ItemWithContent, ItemWithMeta};
+pub use types::{ItemInfo, ItemWithContent, ItemWithMeta};
 pub use utils::{calc_byte_range, extract_tags, parse_comma_tags};