fix: video size error handling — validation, timeouts, failure surfacing

Backend:
- Increase ffmpeg/ffprobe timeouts from 15s to 60s for large videos
- Classify processing errors (timeout, OOM, corrupt) with user-facing messages
- Emit failure details via media:job:update socket event (error field)
- Bump refboard container memory 512M → 1G

Frontend:
- Client-side file size validation (200MB) before upload starts
- Oversized files show immediate error in upload manager, skip upload
- Handle media:job:update status='failed' — surface error in upload panel
- Add processingFailed() to UploadManager for video processing errors

backend/video-utils.js

@@ -34,7 +34,7 @@ function probeVideo(buffer) {
       '-show_format',
       '-show_streams',
       tmpFile,
-    ], { timeout: 15000 }, (err, stdout) => {
+    ], { timeout: 60000 }, (err, stdout) => {
       cleanup(tmpFile, tmpDir);
       if (err) {
         console.warn('[video-utils] ffprobe failed:', err.message);
@@ -91,7 +91,7 @@ function extractPoster(buffer) {
       '-q:v', '3',           // JPEG quality (2=best, 31=worst)
       '-y',                  // overwrite
       tmpOutput,
-    ], { timeout: 15000 }, (err) => {
+    ], { timeout: 60000 }, (err) => {
       if (err) {
         console.warn('[video-utils] ffmpeg poster extraction failed:', err.message);
         cleanup(tmpInput, tmpDir);