Normalize legacy picture imports by item and collection policy (#1336)

Normalize legacy picture imports by item and collection policy (#1336)

* Initial plan

* refactor(importer): normalize picture metadata across all picture importers

- Replace ArtistHelper with TagHelper in phase-02/03 picture importers
- alt_text on ItemImage rows now uses caption-only text (not file path)
- Add photographer to translationExtra in all picture importers
- Remove artist creation from photographer names in all picture importers
- Attach image-type tags to child picture Items when type is nonempty
- Fix isFirstImage in monument-picture-importer to use tracker-based flag
- Replace altParts block with pickBestCaption in phase-06/07 collection picture importers
- Rewrite explore-monument-picture-importer to create child Item(type='picture') records
- Rewrite travels-monument-picture-importer to create child Item(type='picture') records
- Add tests for explore-monument and travels-monument picture importers
- Update existing tests for all modified importers

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: metanull <20630081+metanull@users.noreply.github.com>

* fix: normalize pickBestCaption parameter type to accept undefined captions

Agent-Logs-Url: https://github.com/metanull/inventory-app/sessions/bbc8efd9-dbb2-41af-864e-fb14f6653e5e

Co-authored-by: metanull <20630081+metanull@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: metanull <20630081+metanull@users.noreply.github.com>