dashboard — preview thumbnail par job

dispatcher scp frame_*.jpg (premiere apres trim head) vers
/var/lib/cosma-qc/thumbnails/job_N.jpg a la fin de do_extract.

Endpoint GET /jobs/{id}/thumbnail serve via FileResponse. Template:
<img class=thumb src=jobs/N/thumbnail> si has_thumbnail. 48x27 px,
object-fit cover.

Backfill manuel des jobs deja done (9, 12, 13, 16, 19) via scp direct.

app/templates/_jobs_table.html

@@ -18,6 +18,7 @@
             {% else %}<span class="sq">■</span>{% endif %}
           </span>
           <span class="label">
+            {% if j.has_thumbnail %}<img class="thumb" src="jobs/{{ j.id }}/thumbnail" alt="" loading="lazy">{% endif %}
             <span class="job-id">#{{ j.id }}</span>
             <span class="auv-gp" title="{{ j.gopro_serial }}">{{ j.auv }} {{ j.gp_label }}</span>
             <span class="seg">{{ j.segment_label }}</span>