Fix GSMArena backfill apply after cached dry run

[Seungpyo1007]

Summary

• Apply hash-matched cached GSMArena CONFIRM decisions to source files during --apply, including confirmations saved by an earlier --dry-run.
• Treat an already present URL as success without changing the file, while preserving the guard against changed records and reporting write failures.
• Cover dry-run to apply, repeated apply, and changed worktree behavior in the backfill regression test.

Verification

• python -m pytest tests/verify/test_gsmarena_backfill.py -q --basetemp .pytest_tmp_gsmarena (21 passed)