Add since filter to watched history endpoints

Summary

Add an optional since query parameter to the existing watched history endpoints.

The parameter should filter results by their watch timestamp. The response must keep the existing payload format.

Example

GET /sync/tracking/watched/episodes?since=2026-09-25T14:32:19.646Z

If the only episode watched after that timestamp is Pluribus episode 3, the response should contain only that episode record.

Requirements

• Support since on the existing watched endpoints:

  • /sync/tracking/watched/episodes

  • /sync/tracking/watched/shows

  • /sync/tracking/watched/movies

• Accept an RFC 3339 timestamp.

• Return the existing response format without additional fields.

• Return records whose watch timestamp is later than since.

• Preserve the current behavior when since is omitted.

• Keep existing pagination behavior.

• Support since together with page and limit.

Acceptance criteria

• A request without since returns the current full watched history.

• A request with since returns only records watched after that timestamp.

• The response objects are unchanged.

• Pagination works correctly on filtered results.

• Invalid timestamps return a clear 400 Bad Request.

• The filter works for episode, show, and movie watched history.