Add asset save/loading

[mwtrew]

Status

• Closes https://github.com/RaspberryPiFoundation/digital-editor-issues/issues/1168

Points for consideration:

• See the ADR for performance and security implications.

What's changed?

• Implements show and create for Scratch assets using ActiveStorage.
• Note that authentication has not been implemented in this PR.

[github-actions]

Test coverage

90.13% line coverage reported by SimpleCov.
Run: https://github.com/RaspberryPiFoundation/editor-api/actions/runs/23192276613

[Copilot]

Pull request overview

Implements Scratch asset persistence and retrieval in the API using ActiveStorage, aligning with the referenced ADR for Scratch project storage and enabling save/load of uploaded assets.

Changes:

• Added ScratchAsset model + DB table for storing asset metadata and attaching uploaded files via ActiveStorage.
• Implemented Scratch asset show (redirect to blob) and create (store-once upload) endpoints and updated routes to include optional format.
• Added request specs and a factory for creating Scratch assets with attachments; added middleware + gem to default content-type for binary uploads.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
spec/features/scratch/showing_a_scratch_asset_spec.rb	Removes old placeholder “show SVG” request spec.
spec/features/scratch/creating_and_showing_a_scratch_asset_spec.rb	Adds request specs covering asset create + show redirect behavior.
spec/factories/scratch_assets.rb	Adds a scratch_asset factory with an attached fixture file.
db/schema.rb	Includes new scratch_assets table in schema.
db/migrate/20260310161646_create_scratch_assets.rb	Adds migration to create scratch_assets.
config/routes.rb	Updates Scratch asset create route to accept .:format.
config/application.rb	Inserts middleware to default POST content-type for scratch asset uploads.
app/models/scratch_asset.rb	Introduces ScratchAsset model with attachment and validations.
app/controllers/api/scratch/assets_controller.rb	Implements show redirect and create upload logic for assets.
Gemfile.lock	Adds rack_content_type_default dependency resolution.
Gemfile	Adds rack_content_type_default (and relocates flipper-ui).

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[zetter-rpf]

Fantastic, no suggestions from me.

[mwtrew]

Thanks @zetter-rpf, won't merge just yet as I'd like to see what else is needed for ActiveStorage to use e.g. S3 on staging.

[mwtrew]

Thanks @zetter-rpf, won't merge just yet as I'd like to see what else is needed for ActiveStorage to use e.g. S3 on staging.

Looks like this should work as-is, so I will merge once we've deployed the other stuff and cleared the Staging column.