feat: add streaming tar export/import and rename "none" to "raw"

- Add streaming tar-based export (--export produces .keep.tar)
- Add streaming tar import (--import reads .keep.tar archives)
- Add server endpoints GET /api/export and POST /api/import
- Rename CompressionType::None to CompressionType::Raw with "none" as alias
- Add DB migration to update existing "none" compression values to "raw"
- Fix export endpoint to propagate errors to client instead of swallowing
- Fix import endpoint to return 413 on max_body_size instead of truncating

Export streams items as tar archives without loading entire files into memory.
Import extracts items with new IDs, preserving original order. Both work
locally and via client/server mode.

Co-Authored-By: opencode <noreply@opencode.ai>

src/args.rs

@@ -66,11 +66,11 @@ pub struct ModeArgs {
     pub status_plugins: bool,
 
     #[arg(group("mode"), help_heading("Mode Options"), long, conflicts_with_all(["save", "get", "diff", "list", "delete", "info", "update", "status", "import"]))]
-    #[arg(help("Export an item to data and metadata files (default: latest item)"))]
+    #[arg(help("Export items to a .keep.tar archive (requires IDs or tags)"))]
     pub export: bool,
 
-    #[arg(group("mode"), help_heading("Mode Options"), long, value_name("META_FILE"), conflicts_with_all(["save", "get", "diff", "list", "delete", "info", "update", "status", "export"]))]
-    #[arg(help("Import an item from a metadata file (data from --import-data-file or stdin)"))]
+    #[arg(group("mode"), help_heading("Mode Options"), long, value_name("FILE"), conflicts_with_all(["save", "get", "diff", "list", "delete", "info", "update", "status", "export"]))]
+    #[arg(help("Import items from a .keep.tar archive or legacy .meta.yml file"))]
     pub import: Option<String>,
 
     #[arg(group("mode"), help_heading("Mode Options"), long, conflicts_with_all(["save", "get", "diff", "list", "delete", "info", "update", "status"]))]
@@ -201,14 +201,14 @@ pub struct ItemArgs {
     #[arg(help("Filter string to apply to content when getting items"))]
     pub filters: Option<String>,
 
-    #[arg(
-        help_heading("Export Options"),
-        long,
-        default_value = "{id}_{tags}_{ts}"
-    )]
-    #[arg(help("Template for export filename. Variables: {id} {tags} {ts} {compression}"))]
+    #[arg(help_heading("Export Options"), long, default_value = "{name}_{ts}")]
+    #[arg(help("Template for export tar filename (appends .keep.tar). Variables: {name} {ts}"))]
     pub export_filename_format: String,
 
+    #[arg(help_heading("Export Options"), long, value_name("NAME"))]
+    #[arg(help("Export name used for {name} variable (default: export_<common-tags>)"))]
+    pub export_name: Option<String>,
+
     #[arg(help_heading("Import Options"), long, value_name("DATA_FILE"))]
     #[arg(help("Data file for import (reads from stdin if omitted)"))]
     pub import_data_file: Option<PathBuf>,