VFS/DataProvider/Config refactoring + SSH public key authentication

Phase 1-6 of refactoring plan:
- VFS abstraction (VfsBackend trait + LocalFs + OpenFlags builder)
- DataProvider trait (SqliteProvider + PgProvider, SFTPGo-compatible)
- Config refactoring (AppConfig unified sections, env overrides)
- SSH handlers (sftp/scp/rsync) migrated to VFS + DataProvider
- SSH public key authentication (Ed25519 signature verification)
- SSH stderr → CHANNEL_EXTENDED_DATA support
- Web auth uses DataProvider instead of direct SQL
- User home directory from provider (per-user isolation)
- PostgreSQL auth provider for SFTPGo compatibility

markbase-core/src/provider/mod.rs

@@ -0,0 +1,65 @@
+pub mod sqlite;
+pub mod pg;
+
+use std::path::PathBuf;
+
+/// 用户信息
+#[derive(Debug, Clone)]
+pub struct User {
+    pub username: String,
+    pub password_hash: String,
+    pub home_dir: PathBuf,
+    pub uid: u32,
+    pub gid: u32,
+    pub permissions: String,
+    pub status: i32,
+}
+
+/// Provider 错误类型
+#[derive(Debug)]
+pub enum ProviderError {
+    NotFound(String),
+    AuthFailed(String),
+    Internal(String),
+}
+
+impl std::fmt::Display for ProviderError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ProviderError::NotFound(msg) => write!(f, "Not found: {}", msg),
+            ProviderError::AuthFailed(msg) => write!(f, "Authentication failed: {}", msg),
+            ProviderError::Internal(msg) => write!(f, "Internal error: {}", msg),
+        }
+    }
+}
+
+impl std::error::Error for ProviderError {}
+
+/// 数据提供者 trait（用户认证和配置）
+pub trait DataProvider: Send + Sync {
+    /// 获取用户信息
+    fn get_user(&self, username: &str) -> Result<Option<User>, ProviderError>;
+
+    /// 验证用户密码
+    fn check_password(&self, username: &str, password: &str) -> Result<bool, ProviderError>;
+
+    /// 获取用户主目录
+    fn get_home_dir(&self, username: &str) -> Result<Option<String>, ProviderError>;
+
+    /// 获取用户组列表
+    fn get_user_groups(&self, username: &str) -> Result<Vec<String>, ProviderError> {
+        let _ = username;
+        Ok(Vec::new())
+    }
+
+    /// 检查用户是否存在且启用
+    fn user_exists(&self, username: &str) -> Result<bool, ProviderError> {
+        Ok(self.get_user(username)?.map(|u| u.status == 1).unwrap_or(false))
+    }
+
+    /// 获取用户的公开密钥列表（OpenSSH authorized_keys格式）
+    fn get_public_keys(&self, username: &str) -> Result<Vec<String>, ProviderError> {
+        let _ = username;
+        Ok(Vec::new())
+    }
+}