fix: 3 critical bugs — audio corruption, blob URLs, static duration

1. Audio/Video sync: During playback, let the browser handle media naturally
   and only force-seek when drift > 250ms. Prevents audio glitching caused
   by setting currentTime 30x/sec. When paused/scrubbing, sync precisely.

2. Blob URLs → Server uploads: All media uploads (BrandTabMedia, SceneFieldEditor,
   TemplateFieldInput) now POST to /api/upload and use persistent server URLs
   instead of blob: URLs that vanish on refresh.

3. Dynamic duration: Added useVideoDurations hook that probes actual video
   durations from uploaded form videos. getTemplateDuration and
   compileExpressToTimeline now accept videoDurations to override static
   durationSeconds for form-sourced scenes.

src/components/shared/TemplateFieldInput.tsx

@@ -169,11 +169,18 @@ export const TemplateFieldInput: React.FC<TemplateFieldInputProps> = ({
               type="file"
               accept={isVideoField ? 'video/*' : 'image/*'}
               className="hidden"
-              onChange={(e) => {
+              onChange={async (e) => {
                 const file = e.target.files?.[0];
                 if (file) {
-                  const url = URL.createObjectURL(file);
-                  onChange(url);
+                  const formData = new FormData();
+                  formData.append('file', file);
+                  try {
+                    const res = await fetch('/api/upload', { method: 'POST', body: formData });
+                    const data = await res.json();
+                    if (data.url) onChange(data.url);
+                  } catch (err) {
+                    console.error('Media upload failed:', err);
+                  }
                 }
               }}
             />