Update: adopt _access model for assets, public by default (fixes #77)

[taylortom]

Fixes #77

Update

• Register assets with authored using the default creator grant (previously accessCheck: false) so a creator can always access their own assets.
• Add migration 1.12.0 backfilling existing assets to _access.public: true — assets were a shared pool, so they default to public. New assets inherit public: true from the schema default.

Testing

1. As a non-superuser, GET /api/assets — your own and public assets are returned (previously empty).
2. Start the app against a DB with pre-existing assets; confirm migration 1.12.0 sets _access.public: true on them.

Depends on adapt-security/adapt-authoring-authored#57 (per-item creator grant fix) — should land together.

[github-actions]

🎉 This PR is included in version 1.12.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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