Add compression support to backup workflow

BackupScheduler Enhancement:
- copy_file() now compresses files using ZSTD or LZ4
- min_size threshold: 1024 bytes (smaller files not compressed)
- compression level: 3 (balanced speed/compression)

BackupConfigResponse Updated:
- Added compress, encrypt, include_checksums fields
- compress: 'none' | 'lz4' | 'zstd'
- Default: 'zstd'

REST API Enhancement:
- GET /api/v2/backup/config returns full config
- POST /api/v2/backup/config accepts compression settings

Test Results:
- Set compress='lz4': ✅ Config updated
- Set compress='zstd': ✅ Config updated
- Compression applied via run_backup() (scheduled backup)

Note: Direct create_snapshot API doesn't use compression
(scheduler.run_backup() is the primary backup mechanism)

Build: 495 tests pass

markbase-core/src/vfs/backup_scheduler.rs

@@ -180,14 +180,28 @@ impl BackupScheduler {
     }
 
     fn copy_file(&self, src: &PathBuf, dst: &PathBuf) -> Result<(), VfsError> {
+        use super::compression::Compressor;
+        use super::VfsCompressionConfig;
+        
         let mut src_file = self.backend.open_file(src, &super::open_flags::OpenFlags::new().read())?;
         let data = src_file.read_all()?;
         
+        let final_data = if self.config.compress != super::VfsCompression::None {
+            let compressor = Compressor::new(VfsCompressionConfig {
+                algorithm: self.config.compress.clone(),
+                min_size: 1024,
+                level: 3,
+            });
+            compressor.compress(&data)?
+        } else {
+            data
+        };
+        
         let mut dst_file = self.backend.open_file(
             dst,
             &super::open_flags::OpenFlags::new().write().create().truncate(),
         )?;
-        dst_file.write_all(&data)?;
+        dst_file.write_all(&final_data)?;
         dst_file.flush()?;
 
         Ok(())