[SUBSCRIBER] Implement alert engine in processing pipeline

[ezehf2]

Extend mqtt/subscriber.py or create a new alert_engine.py in the mqtt/ folder that checks each validated reading against the defined thresholds — temperature above 28°C, CO2 above 1000 ppm, noise above 85 dB, occupancy above room capacity. If a threshold is exceeded, call Timi's insert_alert() function to log it in the database. The alert should include the room, sensor type, value, threshold breached, and timestamp. Test by publishing a CO2 reading above 1000 ppm and confirming an alert row appears in the Alerts table. Commit with message [SUBSCRIBER] Add threshold alert engine.