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/modes/client/save.rs

@@ -75,8 +75,8 @@ pub fn mode(
         // Wrap pipe writer with appropriate compression
         let mut compressor: Box<dyn Write> = match compression_type_clone {
             CompressionType::GZip => {
-                use flate2::write::GzEncoder;
                 use flate2::Compression;
+                use flate2::write::GzEncoder;
                 Box::new(GzEncoder::new(pipe_writer, Compression::default()))
             }
             CompressionType::LZ4 => Box::new(lz4_flex::frame::FrameEncoder::new(pipe_writer)),