feat: frame QC scoring + viser per-AUV button

Stage 04 frame extract:
- New lib_frame_qc.py: per-frame Laplacian/contrast/blue-dominance scoring
- Classes: bottom_visible / water_no_bottom / turbid_water / out_of_water
- Sample 1/5 frames after extraction, write qc.json per segment
- Record metrics (frames_total, frames_bottom_visible, bottom_visible_pct)
- Mark job degraded when bottom_visible_pct < 50%

Per-AUV viser view:
- scripts/viser_auv.py loads all PLYs of an AUV, color per file
- POST /pipeline/missions/{id}/auvs/{auv}/view rsyncs ply -> worker
- launches viser on hashed port 9300+, returns URL
- _pipeline.html exposes AUV list, JS handler opens viser tab

app/templates/_pipeline.html

@@ -16,6 +16,16 @@
       {% if m.counts.get('error') %}<span class="cnt err">{{ m.counts.error }} error</span>{% endif %}
     </span>
   </div>
+  {% if m.auvs %}
+  <div class="pm-auvs">
+    <span class="pm-auvs-label">Viser AUV:</span>
+    {% for auv_id in m.auvs %}
+    <button class="btn-viser-auv"
+            data-url="/pipeline/missions/{{ m.id }}/auvs/{{ auv_id }}/view"
+            type="button">{{ auv_id }} ↗</button>
+    {% endfor %}
+  </div>
+  {% endif %}
   <table class="pipeline-jobs-table">
     <thead>
       <tr><th>AUV</th><th>Segment</th><th>Stage</th><th>Status</th><th>Worker</th><th>Duree</th></tr>