/v0/episodes offset 限制应该 -1

若 offset 超出限制，会返回 offset should be less than or equal to {限制数量}。这个限制数量目前是集数，但第一集是 0，所以正确的限制数量是 集数-1。
例：https://api.bgm.tv/v0/episodes?subject_id=454684&limit=1&offset=13 ，返回的 data 是空数组，因为不存在这一集。

Reproduce the /v0/episodes request using the subject_id, limit, and offset values in the example, then locate the endpoint handler. Verify the boundary calculation for zero-based episode numbering and confirm that an offset beyond the final episode returns the expected validation behavior and data response.

Written by the indexing model from the issue text.