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/express/ExpressEditor.tsx

@@ -8,6 +8,7 @@ import { StoryboardView } from './StoryboardView';
 import { SceneFieldEditor } from './SceneFieldEditor';
 import { ExpressStylePanel } from './ExpressStylePanel';
 import { compileExpressToTimeline, getAspectDimensions, getTemplateDuration } from '../../utils/expressCompiler';
+import { useVideoDurations } from '../../hooks/useVideoDurations';
 
 interface ExpressEditorProps {
   designMD: DesignMD;
@@ -62,13 +63,16 @@ export const ExpressEditor: React.FC<ExpressEditorProps> = ({
     setFieldData(prev => ({ ...prev, [fieldId]: value }));
   }, []);
 
+  // Probe actual video durations for form-sourced scenes
+  const videoDurations = useVideoDurations(selectedTemplate, fieldData);
+
   // Compile template to timeline
   const compiled = useMemo(() => {
     if (!selectedTemplate) return null;
-    return compileExpressToTimeline(selectedTemplate, fieldData, designMD, company);
-  }, [selectedTemplate, fieldData, designMD, company]);
+    return compileExpressToTimeline(selectedTemplate, fieldData, designMD, company, videoDurations);
+  }, [selectedTemplate, fieldData, designMD, company, videoDurations]);
 
-  const totalDuration = selectedTemplate ? getTemplateDuration(selectedTemplate) : 0;
+  const totalDuration = selectedTemplate ? getTemplateDuration(selectedTemplate, videoDurations) : 0;
   const fps = 30;
   const totalFrames = Math.max(30, totalDuration * fps);