Title: [DATABASE] Connect Flask dashboard to SQLite database

[ezehf2]

Update src/dashboard/app.py to replace the placeholder_readings dummy data with real queries from Timi's database.py. Import get_latest_readings() and get_active_alerts() from the database module and pass the results to the index template instead of the hardcoded lists. The prepare_readings() function and alerts.py logic should still work — only the data source changes. Test by running the full system with sensors publishing and confirm real readings appear on the dashboard. Commit with message [DASHBOARD] Connect dashboard to live database.