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

static/app.js

@@ -369,7 +369,8 @@ function initStatusPage(jobId, initialStatus) {
   if (previewToggle) {
     previewToggle.addEventListener('change', function() {
       if (this.checked) {
-        livePreviewImg.src = '/api/preview/' + jobId;
+        livePreviewImg.style.display = 'block';
+        livePreviewImg.src = '/api/preview/' + jobId + '?t=' + Date.now();
         if (previewPlaceholder) previewPlaceholder.style.display = '';
         livePreviewSection.style.display = '';
         if (toggleText) toggleText.textContent = 'ON';
@@ -379,6 +380,10 @@ function initStatusPage(jobId, initialStatus) {
         if (previewPlaceholder) previewPlaceholder.style.display = '';
         livePreviewSection.style.display = '';
         if (toggleText) toggleText.textContent = 'OFF';
+        fetch('/api/jobs/' + jobId + '/preview', {
+          method: 'PUT', headers: {'Content-Type': 'application/json'},
+          body: JSON.stringify({enabled: false})
+        });
       }
     });
   }
@@ -552,6 +557,31 @@ function initStatusPage(jobId, initialStatus) {
     if (pollInterval) { clearInterval(pollInterval); pollInterval = null; }
   }
 
+  /* --- Delete button --- */
+  var deleteBtn = document.getElementById('delete-btn');
+  if (deleteBtn) {
+    deleteBtn.addEventListener('click', function() {
+      if (!confirm('Delete this job and all its output files?')) return;
+      deleteBtn.disabled = true;
+      deleteBtn.textContent = 'Deleting...';
+      fetch('/api/jobs/' + jobId, { method: 'DELETE' })
+        .then(function(res) { return res.json(); })
+        .then(function(data) {
+          if (data.status === 'deleted') {
+            window.location.href = '/jobs';
+          } else {
+            deleteBtn.disabled = false;
+            deleteBtn.textContent = 'Delete';
+            if (data.error) alert(data.error);
+          }
+        })
+        .catch(function() {
+          deleteBtn.disabled = false;
+          deleteBtn.textContent = 'Delete';
+        });
+    });
+  }
+
   /* --- Boot --- */
   updateProgressBar(0);
   if (videoThumb) {