Get analytics for multiple posts in one call

Batch version of GET /analytics/posts/{id}. Pass a comma-separated list of numeric OmniSocials post ids in ids and get each post's latest per-platform metrics back in a single request (thread posts are summed across their parts, same as the single-post endpoint). Built for sync clients that would otherwise call the single-post endpoint once per post and hit the 100 requests/minute rate limit. Up to 100 ids per call; page through larger sets in batches. Every requested id is returned (in request order); ids with no collected analytics come back with an empty platforms object rather than being omitted, so clients can map results 1:1 to the ids they sent. Reads stored snapshots from our database, the same data as the single-post endpoint, so it never calls the social platforms live.