feat: add sync checkpoint restore for last successful sync

Capture rolling online DB snapshots after push/drain/reconcile and expose SyncPage 数据恢复 plus checkpoint/restore APIs; mark Z12h and restore done in coop docs.

Co-authored-by: Cursor <cursoragent@cursor.com>

platform/internal/dbsync/manager.go

@@ -120,6 +120,7 @@ func (m *Manager) loop(ctx context.Context, id string) {
 						now := time.Now().UTC()
 						c.LastReconcileAt = &now
 					})
+					ScheduleCheckpoint(m.store, ch, "reconcile")
 				}
 			}
 		}
@@ -176,6 +177,10 @@ func (m *Manager) tick(ctx context.Context, ch *Channel) error {
 			c.LastError = ""
 		}
 	})
+	// 成功且本批有变更 → 防抖打线上库快照（数据恢复）
+	if err == nil && n > 0 {
+		ScheduleCheckpoint(m.store, ch, "drain")
+	}
 	return err
 }