Run App Query

Body for POST /api/v1/apps/{app_id}/query.

Single mode: caller passes optional limit (default 1000, max 5000). Pagination is NOT supported at this endpoint — write LIMIT N OFFSET M directly in your SQL if you need it. Earlier page / page_size params were removed because the engine never threaded page into a real OFFSET (silently returned the same first page_size rows on every page) — see PR #375 review.