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.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014oyi9DMdhALrGz989MNNZJ

config.example.php

@@ -18,10 +18,3 @@ define('COMPLETE_THRESHOLD', 0.95);
 
 // Allow new users to self-register from the login page.
 define('ALLOW_REGISTRATION', true);
-
-// Where generated course thumbnails are cached. Must be writable by the PHP
-// process; created automatically on first scan if missing.
-define('THUMBNAILS_DIR', __DIR__ . '/cache/thumbnails');
-
-// Capture each course's thumbnail this far into its first video (0.0-1.0).
-define('THUMBNAIL_SEEK_PERCENT', 0.4);