feat(mqtt): add native Home Assistant MQTT Discovery integration (SCR-278)

[Starosdev]

Summary

• Add native MQTT Discovery support so Scrutiny drives auto-appear as Home Assistant devices
• Each drive creates 5 HA entities: temperature, health status, power-on hours, power cycle count, and problem binary sensor
• Uses Last Will and Testament (LWT) for automatic offline detection in HA
• Async publishing via goroutines so MQTT never blocks API responses
• Publishes discovery on device registration, state on SMART uploads, and removes devices on archive/delete
• Loads all active devices on startup via LoadInitialData (mirrors existing Prometheus metrics pattern)
• Disabled by default; enable via web.mqtt.enabled: true or SCRUTINY_WEB_MQTT_ENABLED=true

Files Created

File	Purpose
webapp/backend/pkg/mqtt/client.go	MQTT client wrapper (paho, LWT, auto-reconnect)
webapp/backend/pkg/mqtt/discovery.go	HA MQTT Discovery message builders (5 entities per drive)
webapp/backend/pkg/mqtt/publisher.go	High-level publish orchestration
webapp/backend/pkg/mqtt/discovery_test.go	16 discovery tests
webapp/backend/pkg/mqtt/publisher_test.go	7 publisher/state payload tests
webapp/backend/pkg/web/middleware/mqtt.go	Gin middleware to inject publisher
webapp/backend/pkg/web/handler/mqtt_helpers.go	Shared MQTT helper functions for handlers

Files Modified

File	Change
go.mod	Added paho.mqtt.golang v1.5.0 dependency
webapp/backend/pkg/config/config.go	Added web.mqtt.* config defaults
webapp/backend/pkg/web/server.go	AppEngine struct, middleware registration, lifecycle
webapp/backend/pkg/web/handler/upload_device_metrics.go	Publish state after SMART upload
webapp/backend/pkg/web/handler/register_devices.go	Publish discovery on device registration
webapp/backend/pkg/web/handler/delete_device.go	Remove from HA on device deletion
webapp/backend/pkg/web/handler/archive_device.go	Remove from HA on device archive
webapp/backend/pkg/web/handler/unarchive_device.go	Re-publish to HA on device unarchive
example.scrutiny.yaml	Documented MQTT config section

Configuration

web:
  mqtt:
    enabled: false                      # SCRUTINY_WEB_MQTT_ENABLED
    broker: "tcp://localhost:1883"       # SCRUTINY_WEB_MQTT_BROKER
    username: ""                        # SCRUTINY_WEB_MQTT_USERNAME
    password: ""                        # SCRUTINY_WEB_MQTT_PASSWORD
    client_id: "scrutiny"               # SCRUTINY_WEB_MQTT_CLIENT_ID
    topic_prefix: "homeassistant"       # SCRUTINY_WEB_MQTT_TOPIC_PREFIX
    qos: 1                              # SCRUTINY_WEB_MQTT_QOS
    retain: true                        # SCRUTINY_WEB_MQTT_RETAIN

Linked Issues

Closes #278

Test plan

• 23 new unit tests pass (discovery message structure, state payload, status mapping)
• Full test suite passes with zero regressions
• Both scrutiny and collector-metrics binaries build cleanly
• go vet clean (no new warnings)
• Manual integration test with Mosquitto broker and Home Assistant