[DATABASE] Design and commit SQLite schema

[ezehf2]

Create schema.sql in the database/ folder containing the full SQLite database schema. Must include four tables — Rooms (room_id, room_name, capacity), Sensors (sensor_id, room_id, sensor_type, foreign key to Rooms), Sensor_Readings (reading_id, sensor_id, value, unit, timestamp), Alerts (alert_id, room_id, sensor_type, value, threshold, timestamp, resolved). All column names must be lowercase. This file must be committed before database.py so the team can see the structure. Commit with message [DATABASE] Add SQLite schema and reference this issue.