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

README.md

@@ -20,8 +20,7 @@ views switch without page reloads and browser back/forward still work:
 | `#/search/Q` | courses, videos and quizzes matching `Q` |
 | `#/settings` | library folder, preferences, reset |
 
-Only `stream.php` (the mp4 bytes) and `thumb.php` (cached course thumbnails)
-are plain HTTP resources.
+Only `stream.php` (the mp4 bytes) is a plain HTTP resource.
 
 - Recursively scans a videos directory for `.mp4` files and mirrors the folder
   hierarchy (courses → sections → videos, any nesting depth).
@@ -37,8 +36,6 @@ are plain HTTP resources.
   library between **shelves and catalog**; both choices are remembered.
 - **Shelves** group courses by how far you are: in progress, finished, not
   started. The app has no course categories, so progress is what sorts them.
-- **Course thumbnails** — a real frame captured from each course's first
-  video, 40% of the way through it.
 - Seekable streaming via HTTP Range requests; files are addressed by DB id only
   (no path traversal).
 
@@ -74,15 +71,9 @@ Top-level folders become courses on the home page.
      the web server / PHP user).
    - DB credentials (change the password in production).
    - `ALLOW_REGISTRATION`: set `false` once your users have accounts.
-   - `THUMBNAILS_DIR`: where generated course thumbnails are cached — must be
-     *writable* by the web server / PHP user (default: `cache/thumbnails`
-     next to `config.php`).
-   - `THUMBNAIL_SEEK_PERCENT`: how far into a course's first video to capture
-     its thumbnail (default `0.4`, i.e. 40%).
 
-   An `ffmpeg` binary on `$PATH` is required for thumbnails (and is the
-   fallback for video duration if the pure-PHP mp4 parser can't read a file).
-   Without it, courses just keep their colour-block placeholder.
+   No external binaries are needed — PHP with PDO/MySQL is the whole
+   dependency list.
 
 3. **Run** — with the PHP built-in server for testing:
 
@@ -128,29 +119,6 @@ correct set. Best score, last score, and attempt count are stored per user;
 a quiz is *completed* once every question is answered correctly in one
 attempt, and quiz scores count toward folder progress alongside videos.
 
-## Thumbnails
-
-Each course's card is a real frame, not just a colour block: the scanner finds
-the course's first video — in the same order you'd browse to it (subfolders in
-listing order, then that folder's videos) — and asks `ffmpeg` for a frame
-`THUMBNAIL_SEEK_PERCENT` (default 40%) of the way through it, scaled and
-cropped to 640×360. The result is cached at `THUMBNAILS_DIR/<course id>.jpg`
-and served by `thumb.php?course_id=N`, which looks the file up by that ID only
-(same containment check as `stream.php`) and sends normal `ETag`/
-`Last-Modified` caching headers.
-
-- Regeneration only happens when a course's first video has changed since the
-  last scan (tracked via `folders.thumbnail_video_id`), so rescans stay fast.
-- If `ffmpeg` is missing, a video's duration can't be read, or generation
-  otherwise fails, the course simply keeps its colour-block placeholder — the
-  `<img>` 404s and the client removes it (`onerror="this.remove()"`), no error
-  is shown to the user. Failures are logged (`error_log`) and rolled up into
-  the scan's `thumbnails_failed` count.
-- `thumbnail_video_id`/`thumbnail_generated_at` on `folders` are added
-  automatically on first use (`ensureThumbnailColumns()`) if a database
-  created before this feature doesn't have them yet — no manual migration
-  needed, the same self-healing `app_meta` uses.
-
 ## Search
 
 The top-bar box searches as you type (from two characters, debounced) and also
@@ -201,7 +169,7 @@ line up without a separate monospace face.
 
 Courses have no cover art, so each one gets a hue from the design's six-colour
 set (clay, sage, slate, sand, plum, sky), picked from its folder id so it stays
-the same across renders and reloads. That hue drives its thumbnail, its initial
+the same across renders and reloads. That hue drives its card art, its initial
 badge and its progress bar.
 
 anime.js drives the motion: views fade and stagger in, progress bars grow from
@@ -241,9 +209,8 @@ next scan creates it.
 - Folder progress is computed over the folder's entire subtree using a path
   prefix match, with partial credit for videos in progress.
 - Video duration is read at scan time by parsing the mp4's `mvhd` header in
-  pure PHP (ffprobe fallback if the parse fails), so listings show durations
-  before anyone plays the video. The browser still reports the duration on
-  first play as a last-resort fallback.
+  pure PHP, so listings show durations before anyone plays the video. If a file
+  can't be parsed, the browser reports its duration on first play instead.
 
 ## Production notes