content-studio: generate Instagram feed publishable JPEG assets (#593)

[thxforall]

Parent

Parent: #593

What to build

Make Content Studio instagram_feed image assets valid Publishable Assets for Instagram feed publishing by producing public JPEG storage objects.

This task must not add publish UI, call the publish route, call Meta APIs, or add publish status rendering.

Acceptance criteria

• Asset generation converts instagram_feed image assets to JPEG before upload.
• Publishable instagram_feed assets upload with Content-Type: image/jpeg and a .jpg extension.
• PNG, non-JPEG, data:, and fallback preview URLs are not treated as Publishable Assets.
• Existing non-feed asset formats keep their current behavior unless conversion is explicitly needed.
• A focused helper or testable path identifies eligible Instagram feed Publishable Asset URLs for later UI work.
• Unit tests cover JPEG upload metadata, non-JPEG rejection/eligibility, and no regression for non-feed assets.

Blocked by

• content-studio: add Instagram publish Next proxy route (#593) #598

[thxforall]

Closed automatically because PR #612 was merged into dev: #612