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/Cargo.toml

@@ -38,6 +38,7 @@ filetime = "0.2"
 base64 = "0.22"
 tokio = { version = "1", features = ["full"] }
 tokio-postgres = "0.7"
+postgres = "0.19"
 russh = "0.61.2"
 russh-keys = "0.50.0-beta.7"
 russh-sftp = "2.3.0"