fix: toggle preview display bug, add job deletion, optimize pipeline

- Fix: toggle ON now resets img display to block (was stuck on 'waiting')
- Toggle OFF sends API call to stop preview file writes
- Delete button on status page + jobs page cards
- DELETE /api/jobs/<id> endpoint removes job + output files
- PUT /api/jobs/<id>/preview endpoint toggles preview_enabled
- Pipeline skips resize/encode/write when preview_enabled=False
- Reduced preview defaults: 480px max, JPEG quality 60
- 62/62 tests pass

app.py

@@ -255,6 +255,31 @@ def api_cancel_job(job_id):
     return jsonify({"error": "cannot cancel job in state " + job.status.name}), 400
 
 
+@app.route("/api/jobs/<job_id>", methods=["DELETE"])
+def api_delete_job(job_id):
+    """Delete a completed/failed/cancelled job."""
+    success = job_queue.delete_job(job_id)
+    if success:
+        return jsonify({"status": "deleted"})
+    job = job_queue.get_job(job_id)
+    if job is None:
+        return jsonify({"error": "not found"}), 404
+    return jsonify({"error": "cannot delete job in state " + job.status.name}), 400
+
+
+@app.route("/api/jobs/<job_id>/preview", methods=["PUT"])
+def api_toggle_preview(job_id):
+    """Toggle live preview on/off for a running job."""
+    job = job_queue.get_job(job_id)
+    if job is None:
+        return jsonify({"error": "not found"}), 404
+    data = request.get_json(silent=True) or {}
+    enabled = data.get("enabled", True)
+    with job_queue._lock:
+        job.preview_enabled = enabled
+    return jsonify({"status": "ok", "preview_enabled": enabled})
+
+
 @app.route("/api/jobs/<job_id>")
 def api_job_detail(job_id):
     job = job_queue.get_job(job_id)