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

schema.sql

@@ -19,21 +19,15 @@ CREATE TABLE IF NOT EXISTS app_meta (
 
 -- One row per directory found under VIDEOS_ROOT.
 -- `path` is the relative path from VIDEOS_ROOT, e.g. "Course 1/Intro".
--- thumbnail_video_id/thumbnail_generated_at are only ever set on top-level
--- (course) folders — see generateThumbnails() — and have no FK to videos
--- (that would make folders<->videos a circular reference); a stale pointer
--- is harmless and self-corrects on the next scan.
 CREATE TABLE IF NOT EXISTS folders (
-    id                     INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
-    parent_id              INT UNSIGNED NULL,
-    name                   VARCHAR(255) NOT NULL,
-    path                   VARCHAR(700) NOT NULL UNIQUE,
-    depth                  INT UNSIGNED NOT NULL DEFAULT 0,
-    sort_order             INT UNSIGNED NOT NULL DEFAULT 0,
-    is_missing             TINYINT(1) NOT NULL DEFAULT 0,
-    thumbnail_video_id     INT UNSIGNED NULL,
-    thumbnail_generated_at TIMESTAMP NULL,
-    created_at             TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    id         INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+    parent_id  INT UNSIGNED NULL,
+    name       VARCHAR(255) NOT NULL,
+    path       VARCHAR(700) NOT NULL UNIQUE,
+    depth      INT UNSIGNED NOT NULL DEFAULT 0,
+    sort_order INT UNSIGNED NOT NULL DEFAULT 0,
+    is_missing TINYINT(1) NOT NULL DEFAULT 0,
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
     CONSTRAINT fk_folders_parent FOREIGN KEY (parent_id)
         REFERENCES folders(id) ON DELETE CASCADE,
     INDEX idx_folders_parent (parent_id)