Gen2 cameras send eventType=MOVEMENT with eventTags=["PERSON"] for human
detections. The old code did ev.get("eventType") directly, so Gen2 person
events showed the MOVEMENT icon/label/event_type everywhere instead of PERSON.

Add _effective_event_type(event) helper that returns "PERSON" when
eventType=="MOVEMENT" and "PERSON" in (eventTags or []), otherwise the raw
eventType. Applied at both display/routing sites (on_notification FCM path
and cmd_watch polling path) which feed icon selection, Signal alert text,
and webhook event_type. No change to motion-edge routing (sees all events).

Regression test: tests/test_gen2_person_tag.py (11 tests, all branches).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>