Allow opt-in Commons backfill for missing image URL keys

[Seungpyo1007]

Summary

• Add --include-missing-key to include smartphone records without an image_url key; the default still requires an explicit null.
• Insert image fields into missing-key JSON records without changing other content or line endings.
• Cover opt-in eligibility and surgical insertion with offline tests.

Verification

• python -m pytest tests/verify/test_wikipedia_image_backfill.py --basetemp C:/Users/29/AppData/Local/Temp/techengine-wiki-image-pytest -q
• python -m mypy app
• python -m ruff check app tests
• python -m ruff format --check app/verify/wikipedia_image_backfill.py tests/verify/test_wikipedia_image_backfill.py