feat: add export/import modes, unify service layer, fix binary detection

Export/import:
- Add --export and --import modes for both local and client paths
- Use strfmt crate for --export-filename-format templates ({id}, {tags}, {ts}, {compression})
- Import preserves original timestamps via server ?ts= param
- --import-data-file for file-based import; stdin fallback streams with PIPESIZE buffers

Service unification:
- Merge SyncDataService unique methods into ItemService (delete_item now returns Result<Item>)
- Delete AsyncDataService, AsyncItemService, DataService trait (dead code / async-blocking anti-pattern)
- All server handlers use spawn_blocking + ItemService directly
- Extract shared types (ExportMeta, ImportMeta) and helpers (resolve_item_id(s), check_binary_tty)

Binary detection fix:
- Replace broken metadata.get("map") + is_binary(&[]) with actual content sampling
- Both as_meta and allow_binary paths read PIPESIZE sample before deciding
- Never load entire item into memory for binary check

Other fixes:
- Fix lock consistency: all handlers use blocking_lock() in spawn_blocking (no mixed lock().await)
- Use ISO 8601 format for {ts} in export filenames
- Fix resolve_item_ids returning only 1 item for tag lookups
- Fix client get.rs triple-buffering and export.rs whole-file buffering
- Add KeepClient::get_item_content_stream() for streaming reads
- Pass all clippy --features server lints (Path vs PathBuf, &mut conn, etc.)

src/services/mod.rs

@@ -1,25 +1,17 @@
-pub mod async_data_service;
-pub mod async_item_service;
 pub mod compression_service;
-pub mod data_service;
 pub mod error;
 pub mod filter_service;
 pub mod item_service;
 pub mod meta_service;
 pub mod status_service;
-pub mod sync_data_service;
 pub mod types;
 pub mod utils;
 
-pub use async_data_service::AsyncDataService;
-pub use async_item_service::AsyncItemService;
 pub use compression_service::CompressionService;
-pub use data_service::DataService;
 pub use error::CoreError;
 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 sync_data_service::SyncDataService;
 pub use types::{ItemWithContent, ItemWithMeta};
 pub use utils::{calc_byte_range, extract_tags, parse_comma_tags};