Replace the legacy points-to-balance and external points purchase flows
with direct integer balance-to-points exchange. Enforce exchange and
points-transfer limits and capability checks across the UI and REST API.
- P1: remove the duplicate "审核联动" (ai_moderate_background/submit)
block from the attachment permissions page; those settings live in
the "评论审核" / API content audit surfaces.
- P2: rename the "内容审核" submenu to "评论审核" (permissions domain).
- P4: remove the "下载/付费内容中心" page and its schema/route chain;
hardcode runtime policy so download always supports credit purchase
and paid content is limited to posts. Drop the now-dead
cms_features.download_credit flag and parse_slug_list helper.
kate522
