feat(plugin): add plugin interface system and iot-mock implementation

- internal/plugin: Plugin/IoTProvider/ToolProvider/RAGProvider interfaces
- internal/plugin: Registry with type-safe auto-detection, 11 tests
- plugins/iot-mock: 8 simulated smart home devices, 11 tests
- 63 total tests, build + vet clean

backend/internal/plugin/iot.go

@@ -0,0 +1,25 @@
+package plugin
+
+import "context"
+
+// IoTProvider is implemented by plugins that manage IoT devices.
+// Only one IoTProvider can be active at a time.
+type IoTProvider interface {
+	Plugin
+
+	// ListDevices returns all managed devices.
+	ListDevices(ctx context.Context) ([]Device, error)
+
+	// GetDevice returns a single device by ID.
+	GetDevice(ctx context.Context, id string) (*Device, error)
+
+	// SetDevice sets a property on a device (e.g., brightness=80).
+	SetDevice(ctx context.Context, id, field string, value any) error
+
+	// ToggleDevice toggles a device's power state.
+	ToggleDevice(ctx context.Context, id string) error
+
+	// SubscribeChanges returns a channel that emits device state changes.
+	// The channel is closed when the context is cancelled.
+	SubscribeChanges(ctx context.Context) (<-chan DeviceEvent, error)
+}