refactor(BA-5071): Apply RBAC Creator pattern to ImageAlias

[fregataa]

Summary

• Add IMAGE_ALIAS = "image:alias" to RBACElementType enum
• Convert Creator/execute_creator to RBACEntityCreator/execute_rbac_entity_creator in image domain
• Update service, repository, db_source, and tests

Test plan

• pants lint passes
• CI validation

Resolves BA-5071

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📚 Documentation preview 📚: https://sorna--10014.org.readthedocs.build/en/10014/

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📚 Documentation preview 📚: https://sorna-ko--10014.org.readthedocs.build/ko/10014/

[Copilot]

Pull request overview

This PR refactors the image alias creation flow to use the RBAC-scoped entity creation pattern so that creating an ImageAlias also records the appropriate scope→entity association for permissioning.

Changes:

• Added RBACElementType.IMAGE_ALIAS to represent image-alias RBAC entities.
• Updated image alias creation to use RBACEntityCreator / execute_rbac_entity_creator across service, repository, and DB source layers.
• Updated unit test assertions to validate RBAC creator metadata (element type + scope ref).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/unit/manager/services/image/test_image_service.py	Updates unit test to assert RBACEntityCreator usage and correct RBAC scope metadata.
src/ai/backend/manager/services/image/service.py	Switches alias-by-id service logic from Creator to RBACEntityCreator with image scope reference.
src/ai/backend/manager/repositories/image/repository.py	Updates repository method signature to accept RBACEntityCreator for alias creation.
src/ai/backend/manager/repositories/image/db_source/db_source.py	Uses execute_rbac_entity_creator when inserting image aliases, ensuring RBAC association rows are created.
src/ai/backend/common/data/permission/types.py	Adds IMAGE_ALIAS to RBACElementType.

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[Copilot]

RBACElementType.IMAGE_ALIAS is introduced with a value containing : ("image:alias"). This makes RBACElementType values incompatible with the existing ...Id / ...Ref string serialization helpers that use val.partition(":") (e.g., RBACElementRef.from_str() / .to_str()), because round-tripping will break when the enum value itself contains :. Consider either (1) using a colon-free value for RBACElementType (while mapping to EntityType.IMAGE_ALIAS elsewhere), or (2) updating the parsing helpers to split from the right (e.g., rpartition) so element types may safely contain :.