Validate an Instagram location_id

Check whether a Facebook Place ID is a valid Instagram location (a single venue with a precise address) before using it as location_id. Returns { valid: true, name, address } for taggable places; { valid: false, reason } for personal profiles, brand pages, or non-places. If the workspace has no Facebook account (or the app can't read the page), returns { valid: false, unverified: true } — the post's publish step is then the final check.