v1.9.2 — Critical fix: coordinator crash with schedule.* devices

Critical fix: v1.9.0's time-window-aware battery projection crashed the coordinator on every update cycle for any install using a schedule.* helper on a device — all integration sensors went unavailable while the real controlled devices kept running untouched (no switching happened at all, since the coordinator never produced fresh data).

Root cause: the schedule helper's next_event attribute is a native datetime object on the in-memory state (only the REST/websocket APIs serialize it to a string, which is what made it look like a plain string during testing). Calling dt_util.parse_datetime() directly on that datetime raised TypeError: argument must be str every cycle.

Fixed by accepting either a datetime or a string. Same defensive handling applied to sun.sun's next_rising attribute, which had the identical latent risk.

If you're on v1.9.0 or v1.9.1 and your dashboard shows "unavailable"/"nicht verfügbar" for the surplus/battery sensors, update to this version and restart.