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/modes/client/save.rs

@@ -39,7 +39,7 @@ pub fn mode(
     // Determine compression type from settings
     let compression_type = settings_compression_type(cmd, settings);
     let compression_type_str = compression_type.to_string();
-    // In client mode, the client always handles compression (even "none").
+    // In client mode, the client always handles compression (even "raw").
     // The server should never re-compress client data.
     let server_compress = false;