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/api/routes/spoolbuddy.py

@@ -81,6 +81,7 @@ def _device_to_response(device: SpoolBuddyDevice) -> DeviceResponse:
         uptime_s=device.uptime_s,
         update_status=device.update_status,
         update_message=device.update_message,
+        system_stats=json.loads(device.system_stats) if device.system_stats else None,
         online=_is_online(device),
         created_at=device.created_at,
         updated_at=device.updated_at,
@@ -216,6 +217,8 @@ async def device_heartbeat(
         device.nfc_connection = req.nfc_connection
     if req.backend_url:
         device.backend_url = req.backend_url
+    if req.system_stats is not None:
+        device.system_stats = json.dumps(req.system_stats)
 
     # Return and clear pending command
     pending = device.pending_command

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:

backend/app/models/spoolbuddy_device.py

@@ -36,5 +36,6 @@ class SpoolBuddyDevice(Base):
     nfc_ok: Mapped[bool] = mapped_column(Boolean, default=False)
     scale_ok: Mapped[bool] = mapped_column(Boolean, default=False)
     uptime_s: Mapped[int] = mapped_column(Integer, default=0)
+    system_stats: Mapped[str | None] = mapped_column(Text, nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now())
     updated_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now(), onupdate=func.now())

backend/app/schemas/spoolbuddy.py

@@ -44,6 +44,7 @@ class DeviceResponse(BaseModel):
     uptime_s: int
     update_status: str | None = None
     update_message: str | None = None
+    system_stats: dict | None = None
     online: bool = False
     ssh_public_key: str | None = None
     created_at: datetime
@@ -62,6 +63,7 @@ class HeartbeatRequest(BaseModel):
     nfc_reader_type: str | None = None
     nfc_connection: str | None = None
     backend_url: str | None = None
+    system_stats: dict | None = None
 
 
 class HeartbeatResponse(BaseModel):

backend/tests/unit/test_spoolbuddy_system_stats.py

@@ -0,0 +1,118 @@
+"""Tests for SpoolBuddy daemon system_stats collector."""
+
+from unittest.mock import patch
+
+from spoolbuddy.daemon.system_stats import (
+    _cpu_count,
+    _cpu_temp,
+    _disk_info,
+    _load_avg,
+    _memory_info,
+    _os_info,
+    _system_uptime,
+    collect,
+)
+
+
+class TestCpuTemp:
+    def test_reads_thermal_zone(self):
+        with patch("spoolbuddy.daemon.system_stats._read_file", return_value="52100"):
+            assert _cpu_temp() == 52.1
+
+    def test_returns_none_on_missing_file(self):
+        with patch("spoolbuddy.daemon.system_stats._read_file", return_value=None):
+            assert _cpu_temp() is None
+
+    def test_returns_none_on_bad_value(self):
+        with patch("spoolbuddy.daemon.system_stats._read_file", return_value="not_a_number"):
+            assert _cpu_temp() is None
+
+
+class TestMemoryInfo:
+    SAMPLE_MEMINFO = (
+        "MemTotal:        1024000 kB\n"
+        "MemFree:          200000 kB\n"
+        "MemAvailable:     512000 kB\n"
+        "Buffers:           50000 kB\n"
+    )
+
+    def test_parses_meminfo(self):
+        with patch("spoolbuddy.daemon.system_stats._read_file", return_value=self.SAMPLE_MEMINFO):
+            result = _memory_info()
+            assert result is not None
+            assert result["total_mb"] == 1000
+            assert result["available_mb"] == 500
+            assert result["used_mb"] == 500
+            assert result["percent"] == 50.0
+
+    def test_returns_none_on_missing(self):
+        with patch("spoolbuddy.daemon.system_stats._read_file", return_value=None):
+            assert _memory_info() is None
+
+
+class TestDiskInfo:
+    def test_returns_disk_stats(self):
+        result = _disk_info()
+        # Should always work on Linux
+        assert result is not None
+        assert "total_gb" in result
+        assert "used_gb" in result
+        assert "free_gb" in result
+        assert "percent" in result
+        assert 0 <= result["percent"] <= 100
+
+
+class TestLoadAvg:
+    def test_returns_three_values(self):
+        result = _load_avg()
+        assert result is not None
+        assert len(result) == 3
+        for val in result:
+            assert isinstance(val, float)
+
+
+class TestCpuCount:
+    def test_returns_positive_int(self):
+        result = _cpu_count()
+        assert result is not None
+        assert result > 0
+
+
+class TestOsInfo:
+    def test_returns_required_keys(self):
+        result = _os_info()
+        assert "os" in result
+        assert "kernel" in result
+        assert "arch" in result
+        assert "python" in result
+
+    def test_parses_pretty_name(self):
+        fake_release = 'PRETTY_NAME="Raspbian GNU/Linux 12 (bookworm)"\nID=raspbian\n'
+        with patch("spoolbuddy.daemon.system_stats._read_file", return_value=fake_release):
+            result = _os_info()
+            assert result["os"] == "Raspbian GNU/Linux 12 (bookworm)"
+
+
+class TestSystemUptime:
+    def test_parses_uptime(self):
+        with patch("spoolbuddy.daemon.system_stats._read_file", return_value="86400.55 172000.10"):
+            assert _system_uptime() == 86400
+
+    def test_returns_none_on_missing(self):
+        with patch("spoolbuddy.daemon.system_stats._read_file", return_value=None):
+            assert _system_uptime() is None
+
+
+class TestCollect:
+    def test_returns_dict_with_expected_keys(self):
+        result = collect()
+        assert isinstance(result, dict)
+        assert "os" in result
+        # These may or may not be present depending on platform, but os is always present
+
+    def test_all_values_are_json_serializable(self):
+        import json
+
+        result = collect()
+        # Should not raise
+        json.dumps(result)