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

scan.php

@@ -11,5 +11,4 @@ if (PHP_SAPI !== 'cli') {
 $stats = scanLibrary();
 echo "Scan complete: {$stats['folders']} folders, {$stats['videos']} videos, {$stats['quizzes']} quizzes"
    . " ({$stats['missing_folders']} folders / {$stats['missing_videos']} videos"
-   . " / {$stats['missing_quizzes']} quizzes flagged missing)\n"
-   . "Thumbnails: {$stats['thumbnails_generated']} generated, {$stats['thumbnails_failed']} failed\n";
+   . " / {$stats['missing_quizzes']} quizzes flagged missing)\n";