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/db.rs

@@ -292,6 +292,35 @@ pub fn create_item(
     })
 }
 
+/// Creates a new item with a specific timestamp (for import).
+///
+/// # Arguments
+///
+/// * `conn` - Database connection.
+/// * `ts` - Timestamp to use for the item.
+/// * `compression` - Compression type string (e.g., "lz4", "gzip", "none").
+///
+/// # Returns
+///
+/// * `Result<Item>` - The created item with its ID set.
+pub fn insert_item_with_ts(
+    conn: &Connection,
+    ts: chrono::DateTime<chrono::Utc>,
+    compression: &str,
+) -> Result<Item> {
+    let item = Item {
+        id: None,
+        ts,
+        size: None,
+        compression: compression.to_string(),
+    };
+    let item_id = insert_item(conn, item.clone())?;
+    Ok(Item {
+        id: Some(item_id),
+        ..item
+    })
+}
+
 /// Adds a tag to an item.
 ///
 /// Inserts a new tag association in the `tags` table.