feat: harden pipeline UX — preprocess, rembg alpha, remix/lightbox

Downscale to 2000px JPEG before rembg, random blend opacity, timeout
retries, per-variant remix prefill, lightbox, and longer SFTP idle.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/templates/index.html

@@ -9,7 +9,7 @@
         <li class="job-card">
           <a href="/jobs/{{ job.job_id }}">
             {% if job.thumbnail %}
-              <img class="thumb" src="/jobs/{{ job.job_id }}/files/{{ job.thumbnail }}" alt="Variante von Job {{ job.job_id }}" loading="lazy">
+              <img class="thumb lightboxable" src="/jobs/{{ job.job_id }}/files/{{ job.thumbnail }}" alt="Variante von Job {{ job.job_id }}" loading="lazy">
             {% else %}
               <span class="thumb thumb-placeholder status-{{ job.status }}">{{ job.status }}</span>
             {% endif %}