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

[thxforall]

Parent

Parent: #593

What to build

Add the Next.js admin proxy route and web schemas for publishing an approved Instagram Feed Variant through the Rust API.

The route validates the admin session and request shape, then proxies to Rust. It must not call Meta directly and must not use a Supabase fallback for publishing.

Acceptance criteria

• Web schemas include the publish request/response shape using imageAssetUrl, not arbitrary mediaUrl or accountId.
• POST /api/v1/content/variants/[id]/publish/instagram requires an admin session.
• Non-admin users receive 403.
• Missing API_BASE_URL returns 503 with a clear Rust API required message.
• When API_BASE_URL exists, the route proxies to Rust with the user bearer token.
• The route does not call Meta directly.
• The route does not fall back to Supabase for publishing.
• Route tests cover admin requirement, 403, invalid request, successful proxy, upstream error passthrough, and missing API base.

Blocked by

• content-studio: implement Rust Instagram publish service (#593) #597

[thxforall]

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