content-studio: add Instagram publish UI/status controls (#593)

[thxforall]

Parent

Parent: #593

What to build

Add the Content Studio UI path for publishing approved Instagram Feed Variants and showing the latest Publish Job outcome.

This task consumes the publishable JPEG asset support from #600. It must not change asset generation, DB schema, Rust publish behavior, or Meta API behavior.

Acceptance criteria

• The publish button is visible only for approved instagram_feed variants with a publishable JPEG asset.
• The UI calls the Next proxy route with imageAssetUrl as a Content Studio Publishable Asset URL.
• Request-in-flight state is shown as client loading state.
• Latest persisted Publish Job is read from publishJobsByVariantId[variant.id].
• Published jobs show the published media id.
• If the latest Publish Job is published, the publish button is disabled or replaced with an already-published state; force republish is not exposed.
• Failed jobs show a useful failure reason from error_json.message, with a safe fallback when missing.
• The global manual publish only label is replaced with state-aware publish configuration/status copy.
• UI/unit tests cover button visibility, already-published disabled state, asset eligibility, loading, success, and failure rendering.

Blocked by

• content-studio: add Instagram publish Next proxy route (#593) #598
• content-studio: generate Instagram feed publishable JPEG assets (#593) #600

[thxforall]

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