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.

scripts/dispatcher.py

@@ -379,6 +379,25 @@ def do_extract(job: sqlite3.Row, worker: dict) -> str:
         print(f"  ↳ job #{job['id']}: only {remaining} underwater frames (<{min_frames}) — marking skipped")
         set_status(job["id"], status="skipped", error=f"too short: {remaining} underwater frames")
         raise RuntimeError("skipped_short")
+    # Copy the first kept frame back to the dashboard host as a thumbnail so the UI can show
+    # what the job actually sees. Mid-point would be better but first is cheap and on disk now.
+    thumb_dir = DB_PATH.parent / "thumbnails"
+    thumb_dir.mkdir(exist_ok=True)
+    thumb_local = thumb_dir / f"job_{job['id']}.jpg"
+    subprocess.run(
+        ["scp", "-o", "BatchMode=yes",
+         f"{worker['ssh_alias']}:{frames_dir}/frame_*.jpg", str(thumb_local)],
+        capture_output=True, timeout=30,
+    )  # globbing only picks 1 on the remote side via shell expansion
+    # If glob scp is empty, try first explicit by listing
+    if not thumb_local.exists() or thumb_local.stat().st_size == 0:
+        rc, out, _ = ssh(worker["ssh_alias"], f"ls {shlex.quote(frames_dir)}/frame_*.jpg 2>/dev/null | head -1")
+        first = out.strip()
+        if first:
+            subprocess.run(
+                ["scp", "-o", "BatchMode=yes", f"{worker['ssh_alias']}:{first}", str(thumb_local)],
+                capture_output=True, timeout=30,
+            )
     # Trim once per job so LVM thin pool on the host actually reclaims the freed blocks.
     ssh(worker["ssh_alias"], "sudo fstrim / 2>/dev/null || fstrim / 2>/dev/null", timeout=60)
     return frames_dir