Add Backup REST API endpoints (Phase 5-6)

REST API Implementation:
- 8 backup/snapshot endpoints added to server.rs
- BackupScheduler: add get_config()/set_config() methods

Endpoints:
- GET /api/v2/backup/stats - Scheduler status
- GET/POST /api/v2/backup/config - Config management
- POST /api/v2/backup/run - Manual backup trigger
- GET /api/v2/snapshots - List snapshots
- POST/DELETE /api/v2/snapshots/:name - Create/delete snapshot
- POST /api/v2/snapshots/:name/restore - Restore snapshot
- GET /api/v2/storage/stats - Storage metrics

Test Results:
- curl /api/v2/backup/stats ✅
- curl /api/v2/backup/config ✅
- curl /api/v2/storage/stats ✅
- curl /api/v2/snapshots ✅

Build: 495 tests pass
Server: Port 11438 running with new endpoints

markbase-core/src/vfs/backup_scheduler.rs

@@ -77,6 +77,17 @@ impl BackupScheduler {
         self.config.enabled
     }
 
+    pub fn get_config(&self) -> &BackupScheduleConfig {
+        &self.config
+    }
+
+    pub fn set_config(&mut self, config: BackupScheduleConfig) {
+        self.config = config;
+        if self.config.enabled {
+            self.schedule_next();
+        }
+    }
+
     pub fn schedule_next(&mut self) {
         let now = current_time_secs();
         let interval_secs = self.config.interval_hours * 3600;