Enhanced MQTT support with separate topics and multipliers (#173)

- Add separate MQTT topics for power, energy, and state monitoring
  - mqtt_power_topic, mqtt_power_path, mqtt_power_multiplier
  - mqtt_energy_topic, mqtt_energy_path, mqtt_energy_multiplier
  - mqtt_state_topic, mqtt_state_path, mqtt_state_on_value
- Support different MQTT topics per data type (e.g., Zigbee2MQTT with
  separate power/energy/state topics)
- Individual multipliers for power and energy (e.g., mW→W, Wh→kWh)
- Configurable ON value for state monitoring (e.g., "ON", "true", "1")
- Maintain backward compatibility with legacy mqtt_topic/mqtt_multiplier
- Database migration auto-copies legacy fields to new fields
- Update backup/restore to handle new MQTT fields
- Add backend tests for new MQTT configurations
- Update frontend form with organized Power/Energy/State sections

Closes #173

frontend/src/__tests__/components/SmartPlugCard.test.tsx

@@ -21,12 +21,24 @@ const createMockPlug = (overrides: Partial<SmartPlug> = {}): SmartPlug => ({
   plug_type: 'tasmota',
   ip_address: '192.168.1.100',
   ha_entity_id: null,
-  // MQTT fields
+  ha_power_entity: null,
+  ha_energy_today_entity: null,
+  ha_energy_total_entity: null,
+  // MQTT fields (legacy)
   mqtt_topic: null,
-  mqtt_power_path: null,
-  mqtt_energy_path: null,
-  mqtt_state_path: null,
   mqtt_multiplier: 1.0,
+  // MQTT power fields
+  mqtt_power_topic: null,
+  mqtt_power_path: null,
+  mqtt_power_multiplier: 1.0,
+  // MQTT energy fields
+  mqtt_energy_topic: null,
+  mqtt_energy_path: null,
+  mqtt_energy_multiplier: 1.0,
+  // MQTT state fields
+  mqtt_state_topic: null,
+  mqtt_state_path: null,
+  mqtt_state_on_value: null,
   printer_id: 1,
   enabled: true,
   auto_on: true,