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/server.rs

@@ -2750,6 +2750,9 @@ pub struct BackupConfigResponse {
     pub interval_hours: u64,
     pub max_snapshots: usize,
     pub auto_cleanup: bool,
+    pub compress: String,
+    pub encrypt: bool,
+    pub include_checksums: bool,
 }
 
 #[derive(Debug, Serialize, Deserialize)]
@@ -2790,24 +2793,37 @@ async fn get_backup_stats_handler() -> Json<BackupStatsResponse> {
 async fn get_backup_config_handler() -> Json<BackupConfigResponse> {
     let scheduler = BACKUP_SCHEDULER.lock().unwrap();
     let config = scheduler.get_config();
+    let compress_name = match config.compress {
+        crate::vfs::VfsCompression::None => "none",
+        crate::vfs::VfsCompression::Lz4 => "lz4",
+        crate::vfs::VfsCompression::Zstd => "zstd",
+    };
     Json(BackupConfigResponse {
         enabled: config.enabled,
         interval_hours: config.interval_hours,
         max_snapshots: config.max_snapshots,
         auto_cleanup: config.auto_cleanup,
+        compress: compress_name.to_string(),
+        encrypt: config.encrypt,
+        include_checksums: config.include_checksums,
     })
 }
 
 async fn set_backup_config_handler(Json(config): Json<BackupConfigResponse>) -> Json<serde_json::Value> {
     let mut scheduler = BACKUP_SCHEDULER.lock().unwrap();
+    let compress = match config.compress.as_str() {
+        "lz4" => crate::vfs::VfsCompression::Lz4,
+        "zstd" => crate::vfs::VfsCompression::Zstd,
+        _ => crate::vfs::VfsCompression::None,
+    };
     let new_config = BackupScheduleConfig {
         enabled: config.enabled,
         interval_hours: config.interval_hours,
         max_snapshots: config.max_snapshots,
         auto_cleanup: config.auto_cleanup,
-        compress: scheduler.get_config().compress.clone(),
-        encrypt: scheduler.get_config().encrypt,
-        include_checksums: scheduler.get_config().include_checksums,
+        compress,
+        encrypt: config.encrypt,
+        include_checksums: config.include_checksums,
     };
     scheduler.set_config(new_config);
     Json(serde_json::json!({"success": true, "message": "Backup config updated"}))