Adds api_client support for GET /licenses/{key}/schedule with If-None-Match
and target-payload parsing (version/type/daily/weekly + legacy blocks), an
atomic on-disk schedule cache, a pure schedule engine (half-open blocks,
midnight self-wrap for daily, Sunday->Monday spillover for weekly, invalid
blocks skipped with a warning), and a background ScheduleManager that loads
the cache offline and pulls on boot and every 5 minutes. Adds engine/cache
tests against the shared docs/fixtures, including 304, corrupt cache, and
offline cases.
