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

assets/style.css

@@ -568,17 +568,6 @@ button, input { font-family: inherit; font-size: inherit; }
 }
 .thumb::before { top: -30%; right: -12%; width: 62%; background: rgba(255, 255, 255, .2); }
 .thumb::after { left: -8%; bottom: -32%; width: 48%; background: rgba(16, 15, 13, .14); }
-/* A captured frame sits above the decorative blobs (opaque, so it fully
-   hides them once loaded) but below the text/icon overlays. onerror removes
-   this element on a 404, leaving the colour-block placeholder untouched. */
-.thumb img {
-  position: absolute;
-  inset: 0;
-  width: 100%;
-  height: 100%;
-  object-fit: cover;
-  z-index: 1;
-}
 .thumb-cat {
   position: absolute;
   left: 10px; top: 8px;