Locally stored thumbnails for video and YouTube uploads

- Server-side video thumbnails: ffmpeg-static grabs a frame at ~1s,
  scales it to 480px and stores it under /uploads/thumbs/ at upload
  time (direct files, remote URLs and comment attachments)
- YouTube thumbnails are downloaded once at upload time instead of
  being hotlinked from i.ytimg.com on every render; remote URL kept
  as fallback for legacy rows
- thumbUrl now covers video/audio/youtube/twitch in grid meta,
  profile uploads and notifications
- Generated thumbnail files are removed on single/bulk delete

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

server/api/uploads/[id].delete.ts

@@ -12,7 +12,7 @@ export default defineEventHandler(async (event) => {
   const db = useDb()
 
   const [upload] = await db
-    .select({ fileName: uploads.fileName })
+    .select({ fileName: uploads.fileName, previewImage: uploads.previewImage })
     .from(uploads)
     .where(eq(uploads.id, id))
     .limit(1)
@@ -30,11 +30,15 @@ export default defineEventHandler(async (event) => {
   if (upload.fileName) {
     await unlink(join(uploadDir(), upload.fileName)).catch(() => {})
   }
+  if (upload.previewImage?.startsWith('/uploads/')) {
+    await unlink(join(process.cwd(), 'public', upload.previewImage)).catch(() => {})
+  }
   for (const f of attachmentFiles) {
     await unlink(join(uploadDir(), 'attachments', f.fileName)).catch(() => {})
     if (f.cover) {
       await unlink(join(process.cwd(), 'public', f.cover)).catch(() => {})
     }
   }
+  broadcastToAll({ type: 'upload-deleted', ids: [id] })
   return { ok: true }
 })