Add annotation UI for building a custom training dataset

New page /event/{id}/annotate:
- Frame selector grid (all frames from the event)
- Canvas with click-and-drag rectangle drawing to mark the plate bbox
- Touch support for tablets
- Plate text input (pre-filled with auto-detected plate)
- Saves image + YOLO label (.txt) + plates.csv to /data/annotations/
- Stores annotation metadata in DB (annotations table)

/dataset/export serves a ZIP with images/, labels/, plates.csv and
a data.yaml ready for YOLOv8/v9 fine-tuning on Google Colab.

"Annoter" button added to event detail header.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/templates/event_detail.html

@@ -35,6 +35,7 @@
     <a href="/" class="btn-ghost">← Retour</a>
     <span class="text-xl">🚗</span>
     <span class="font-bold flex-1">{{ ev.time_str }}</span>
+    <a href="/event/{{ ev.id }}/annotate" class="btn-ghost">✏ Annoter</a>
     {% if ev.plate %}
     {% if is_whitelisted %}
     <form action="/whitelist/remove/{{ ev.plate }}" method="post">