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/lib.rs

@@ -23,6 +23,8 @@ pub mod import_markdown;  // Category View Module - 双视图管理（Phase 1）
 // pub mod ssh2_mod;  // ssh2辅助模块（已禁用）
 pub mod ssh_server;  // SSH服务器（Phase 1-9完成，正在修复编译错误）⭐⭐⭐⭐⭐
 pub mod sync;
+pub mod provider;  // DataProvider抽象层（Phase 5）
+pub mod vfs;  // VFS抽象层（Phase 1-6重构计划）
 
 // Re-export from external filetree crate
 pub use filetree::node::FileNode;