Implement night mode support and improve startup sequence

[penguineer]

This pull request adds a "nightmode" feature to the LED control service, allowing the LEDs to be turned off at night except for a brief period after a state change. The implementation introduces new MQTT topic handling, state tracking, and timing logic to manage visibility during nightmode. It also improves startup and shutdown behavior for the LEDs.

Fixes #32

Nightmode feature and MQTT integration:

• Added support for a new MQTT topic (MQTT_NIGHTMODE_TOPIC) to enable or disable nightmode via MQTT messages. When nightmode is active, LEDs remain off except for a short visibility window after a state change. [1] [2]
• Updated the MQTT message callback to handle both the ampel state topic and the new nightmode topic, and to manage the requested LED state and visibility timer accordingly. [1] [2]

Nightmode state management and timing:

• Introduced global variables to track nightmode status, requested LED state, and a monotonic timer for brief visibility during nightmode. Added a helper function to get the current monotonic time. [1] [2]
• Implemented the apply_physical_state function to control LED output based on nightmode and timer state, ensuring LEDs are only shown briefly in nightmode.
• In the main loop, added logic to turn off LEDs automatically when the nightmode visibility timer expires.

Startup and shutdown improvements:

• Added a startup sequence to briefly show each LED color for one second when the service starts.
• Ensured LEDs are turned off on service shutdown for safety.

[penguineer]

Somehow with this solution the status is not regained after rebooting wittgenstein or restarting the service. This worked before on master.

Needs further investigation.