feat(alerts): show lifecycle status labels using NWS messageType

[Orinks]

Closes #332

What changed

Adds lifecycle status labels to alert list items — (New), (Updated) — so users can see at a glance what's changed since the last poll.

Approach

Labels are driven directly by the NWS API's messageType field on each alert:

messageType	Label
Alert	(New) — freshly issued by NWS
Update	(Updated) — NWS explicitly amended it
Cancel	(no label — alert no longer active)

Visual Crossing alerts get no label — VC doesn't provide a messageType equivalent; it repackages NWS data with less metadata, so labels would be unreliable.

Why not snapshot diffing?

An earlier approach used before/after snapshot comparison to derive labels. That had edge cases: labels were wrong on first launch (everything appeared New), broke on app restart, and misclassified amended alerts that NWS re-issues with a new ID. Using messageType directly fixes all of these — it reflects NWS's intent, is stateless, and survives restarts.

Note: the diff infrastructure (diff_alerts, AlertLifecycleDiff) is kept — it still drives the notification system (sounds/toasts for escalations, cancellations, etc.), which is a separate concern.

Files changed

• models/alerts.py — added message_type: str | None field to WeatherAlert
• weather_client_nws.py — pass messageType from NWS props through to the model
• alert_lifecycle.py — compute_lifecycle_labels now takes list[WeatherAlert] instead of a diff
• ui/main_window.py — updated caller; label computation is now fully stateless
• tests/test_alert_lifecycle.py — updated tests for new signature and NWS/VC behavior