Filter cache runs by execution date range

The cache runs listing endpoint now accepts field-scoped date filters so reporting and hit-rate tools can request runs within a specific execution window. Use ran_at[gt], ran_at[gte], ran_at[lt], or ran_at[lte] with Unix timestamps in seconds to bound the result set without paginating the full history. Invalid timestamp values return a bad request with a concrete message, and cache run responses now serialize ran_at from its stored value instead of deriving it from created_at. Existing git reference, branch, commit, and pagination filters keep working alongside the new date filters.