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/cli/tools/smb_server.rs

@@ -0,0 +1,71 @@
+use clap::Subcommand;
+
+#[derive(Subcommand)]
+pub enum SmbServerCommand {
+    #[command(name = "smb-start")]
+    Start {
+        #[arg(short, long, default_value = "4445")]
+        port: u16,
+
+        #[arg(short, long, default_value = "/Users/accusys/momentry/var/sftpgo/data/demo")]
+        root: String,
+
+        #[arg(short, long, default_value = "markbase")]
+        share_name: String,
+
+        #[arg(long)]
+        read_only: bool,
+    },
+}
+
+pub async fn handle_smb_server_command(cmd: SmbServerCommand) -> anyhow::Result<()> {
+    #[cfg(feature = "smb-server")]
+    {
+        match cmd {
+            SmbServerCommand::Start { port, root, share_name, read_only } => {
+                use std::path::PathBuf;
+
+                use smb_server::{Access, Share, SmbServer};
+                use tracing_subscriber::EnvFilter;
+
+                let _ = tracing_subscriber::fmt()
+                    .with_env_filter(
+                        EnvFilter::try_from_default_env()
+                            .unwrap_or_else(|_| EnvFilter::new("info")),
+                    )
+                    .try_init();
+
+                let addr: std::net::SocketAddr =
+                    format!("0.0.0.0:{}", port).parse()?;
+                let root_path = PathBuf::from(&root);
+
+                let vfs = Box::new(crate::vfs::local_fs::LocalFs::new());
+                let backend = crate::vfs::smb_server_backend::VfsShareBackend::new(vfs, root_path)
+                    .read_only(read_only);
+
+                let share = Share::new(&share_name, backend)
+                    .user("demo", Access::ReadWrite);
+
+                let server = SmbServer::builder()
+                    .listen(addr)
+                    .user("demo", "demo123")
+                    .share(share)
+                    .build()?;
+
+                log::info!("SMB server listening on {}", addr);
+                log::info!("Share '{}' at root: {}", share_name, root);
+                log::info!("User: demo / demo123");
+
+                server.serve().await?;
+            }
+        }
+    }
+
+    #[cfg(not(feature = "smb-server"))]
+    {
+        let _ = cmd;
+        anyhow::bail!("SMB server support not enabled. Build with --features smb-server");
+    }
+
+    Ok(())
+}