SMB Server Phase 2: VFS backend build fix + integration test

- Add VfsFile: Send supertrait for Mutex compatibility
- Fix SmbServerCommand: struct → Subcommand enum with Start variant
- Fix tracing_subscriber::init() → try_init() to avoid panic when
  logger already initialized
- Fix CLI subcommand name: smb-server → smb-start (flatten naming)
- Add #[command(name = "smb-start")] for CLI disambiguation
- Fix unused variable warnings (smb_fs.rs, smb_server_backend.rs)
- Remove unused VfsFile imports (webdav.rs, scp_handler.rs)
- Integration test: Docker smbclient verified (list, upload, read)

markbase-core/src/vfs/mod.rs

@@ -1,6 +1,9 @@
 pub mod local_fs;
 pub mod open_flags;
 pub mod s3_fs;
+pub mod smb_fs;
+#[cfg(feature = "smb-server")]
+pub mod smb_server_backend;
 pub mod util;
 
 use std::path::{Path, PathBuf};
@@ -81,7 +84,7 @@ pub struct VfsDirEntry {
 }
 
 /// 打开文件的抽象
-pub trait VfsFile {
+pub trait VfsFile: Send {
     fn read(&mut self, buf: &mut [u8]) -> Result<usize, VfsError>;
     fn write(&mut self, buf: &[u8]) -> Result<usize, VfsError>;
     fn seek(&mut self, pos: std::io::SeekFrom) -> Result<u64, VfsError>;