Add software brightness control for Adalight devices

Emulates hardware brightness by multiplying pixel values before serial
send. Stored per-device and persisted across restarts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/main.py

@@ -157,6 +157,7 @@ async def lifespan(app: FastAPI):
                 calibration=device.calibration,
                 device_type=device.device_type,
                 baud_rate=device.baud_rate,
+                software_brightness=device.software_brightness,
             )
             logger.info(f"Registered device: {device.name} ({device.id})")
         except Exception as e: