[DATABASE] Add historical data query for dashboard charts

[ezehf2]

Add a new function get_historical_readings(room_id, sensor_type, limit=50) to database.py that returns the last N readings for a given room and sensor type ordered by timestamp. This function will be used by the Flask dashboard to populate the historical charts in Sprint 3. Test the function locally by inserting sample data and confirming it returns the correct rows in the correct order. Commit with message [DATABASE] Add historical data retrieval function.