feat(backup): add pre-migration backup, periodic backup, backfill warning, and backup management UI

Four improvements to the database backup mechanism:

1. Auto backup before schema migration - creates a snapshot when
   upgrading from an older database version, providing a safety net
   beyond the existing SAVEPOINT rollback mechanism.

2. Periodic startup backup - checks on app launch whether the latest
   backup is older than 24 hours and creates a new one if needed,
   ensuring all users have recent backups regardless of usage patterns.

3. Backfill failure notification - switch now returns SwitchResult with
   warnings instead of silently ignoring backfill errors, so users are
   informed when their manual config changes may not have been saved.

4. Backup management UI - new BackupListSection in Settings > Data
   Management showing all backup snapshots with restore capability,
   including a confirmation dialog and automatic safety backup before
   restore.

src/hooks/useBackupManager.ts

@@ -0,0 +1,32 @@
+import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
+import { backupsApi } from "@/lib/api";
+
+export function useBackupManager() {
+  const queryClient = useQueryClient();
+
+  const {
+    data: backups = [],
+    isLoading,
+    refetch,
+  } = useQuery({
+    queryKey: ["db-backups"],
+    queryFn: () => backupsApi.listDbBackups(),
+  });
+
+  const restoreMutation = useMutation({
+    mutationFn: (filename: string) => backupsApi.restoreDbBackup(filename),
+    onSuccess: async () => {
+      // Invalidate all queries to refresh data from restored database
+      await queryClient.invalidateQueries();
+      // Refetch backup list
+      await refetch();
+    },
+  });
+
+  return {
+    backups,
+    isLoading,
+    restore: restoreMutation.mutateAsync,
+    isRestoring: restoreMutation.isPending,
+  };
+}