Remove course thumbnails and the ffmpeg dependency

Course cards go back to the colour-block art that previously sat under the
captured frame, so the UI is unchanged apart from losing the photo.

- Drop thumb.php, generateThumbnails(), captureThumbnail() and
  ensureThumbnailColumns(), plus the first-video tracking the scanner only
  kept to pick a frame source.
- Drop the ffprobe fallback in probeDuration(); the pure-PHP mvhd parse is
  now the only scan-time probe, with the player's report on first play as
  the remaining fallback.
- Drop THUMBNAILS_DIR/THUMBNAIL_SEEK_PERCENT from config, the
  thumbnail_* columns from schema.sql, the thumbnail counters from scan
  output, .thumb img from the stylesheet, and the matching README parts.

Existing databases keep two now-unused folders.thumbnail_* columns; they
are inert and can be dropped by hand if wanted.

Claude-Session: https://claude.ai/code/session_014oyi9DMdhALrGz989MNNZJ

api/progress.php

@@ -55,7 +55,8 @@ $stmt = $db->prepare(
 );
 $stmt->execute([$userId, $videoId, $position, $duration > 0 ? $duration : null, $completed]);
 
-// Cache the duration on the video row so listings can show it without ffprobe.
+// Cache the duration on the video row so listings can show it without waiting
+// for someone to play the video (and for files the scanner couldn't parse).
 if ($duration > 0) {
     $db->prepare('UPDATE videos SET duration_seconds = ? WHERE id = ? AND duration_seconds IS NULL')
        ->execute([$duration, $videoId]);