Add SpoolBuddy System tab with live OS stats from Raspberry Pi

The daemon now collects CPU temp, core count, load average, memory/disk
  usage, OS info, and system uptime every heartbeat using stdlib-only reads
  from /proc and /sys. Stats are sent as a JSON blob in the heartbeat
  payload, stored in a new system_stats TEXT column, and displayed in a
  new "System" tab in SpoolBuddy Settings with color-coded usage bars.

backend/app/core/database.py

@@ -1424,6 +1424,12 @@ async def run_migrations(conn):
     except OperationalError:
         pass  # Already applied
 
+    # Migration: Add system_stats JSON blob column to spoolbuddy_devices
+    try:
+        await conn.execute(text("ALTER TABLE spoolbuddy_devices ADD COLUMN system_stats TEXT"))
+    except OperationalError:
+        pass  # Already applied
+
     # Migration: Convert ams_labels table from (printer_id, ams_id) key to ams_serial_number key
     # Labels are now keyed by AMS serial number so they persist when the AMS is moved to another printer.
     try: