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/webdav.rs

@@ -1,5 +1,5 @@
 use crate::vfs::open_flags::OpenFlags;
-use crate::vfs::{VfsBackend, VfsDirEntry, VfsStat, VfsFile};
+use crate::vfs::{VfsBackend, VfsDirEntry, VfsStat};
 use crate::ssh_server::upload_hook::UploadHook;
 use bytes::{Buf, Bytes};
 use dav_server::davpath::DavPath;