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/brand/BrandTabMedia.tsx

@@ -131,9 +131,16 @@ export const BrandTabMedia: React.FC<BrandTabMediaProps> = ({ designMD, handleDe
               compact
               accept="audio/*"
               label="Subir audio"
-              onFiles={(files) => {
-                const url = URL.createObjectURL(files[0]);
-                handleDesignChange('brandAudioUrl', url);
+              onFiles={async (files) => {
+                const formData = new FormData();
+                formData.append('file', files[0]);
+                try {
+                  const res = await fetch('/api/upload', { method: 'POST', body: formData });
+                  const data = await res.json();
+                  if (data.url) handleDesignChange('brandAudioUrl', data.url);
+                } catch (err) {
+                  console.error('Audio upload failed:', err);
+                }
               }}
             />
           </div>
@@ -231,9 +238,16 @@ const VideoUploadSimple: React.FC<{
             compact
             accept="video/*"
             label="Subir archivo"
-            onFiles={(files) => {
-              const url = URL.createObjectURL(files[0]);
-              onUrlChange(url);
+            onFiles={async (files) => {
+              const formData = new FormData();
+              formData.append('file', files[0]);
+              try {
+                const res = await fetch('/api/upload', { method: 'POST', body: formData });
+                const data = await res.json();
+                if (data.url) onUrlChange(data.url);
+              } catch (err) {
+                console.error('Video upload failed:', err);
+              }
             }}
           />
         </div>