fix: download trailers as separate video+audio and merge with ffmpeg (v1.0.0.10)

YouTube stopped serving muxed streams and YoutubeExplode 6.6.0 failed on
every video ("Video is not available"), so no trailer downloaded. Bump
YoutubeExplode to 6.6.2, download the best video-only + audio-only streams
and stream-copy them into one MP4 with ffmpeg. The separate files are
always deleted afterwards, merge or not.

Also delete the trailer folder when a download fails. Empty folders were
treated as "already downloaded" (never retried) and counted toward
MaxTotalTrailers, so over time they pushed out every real trailer. The
skip check now requires the trailer file itself.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

build.yaml

@@ -1,5 +1,5 @@
 ---
-version: 1.0.0.9
+version: 1.0.0.10
 name: CinemaTrailers4Jellyfins
 guid: b581493e-1046-40ed-b6dc-cb8027624984
 description: >
@@ -12,10 +12,13 @@ category: General
 owner: 514mart
 targetAbi: 10.11.0.0
 changelog:
-  - Skip trailers and pre-roll/feature pre-roll bumpers the current user
-    can't actually access (e.g. a library not in their library access list)
-    — some Cinema Mode clients hang indefinitely trying to play an
-    inaccessible item, so these are now filtered out per-user instead
+  - Fix trailer downloads, broken by a YouTube change. YouTube no longer
+    serves combined video+audio files, so the built-in downloader now fetches
+    the video and audio separately, merges them with ffmpeg (no re-encode)
+    and deletes the separate files afterwards
+  - A failed download no longer leaves an empty trailer folder behind.
+    Empty folders counted toward "Max trailers to keep" (pushing out real
+    trailers) and were never retried
 
 dotnetProjects:
   - name: Jellyfin.Plugin.CinemaTrailers4Jellyfins