fix: use jellyfin-ffmpeg and explicit -f mp4 for fake-movie generation

Two issues prevented fake-movie generation:
- ffmpeg was invoked by bare name, but only /usr/lib/jellyfin-ffmpeg/ffmpeg
  exists on the host (Jellyfin's bundled copy, not on PATH). Now probing that
  path first with fallback to PATH.
- The temp file extension .tmp caused ffmpeg to fail with "Unable to choose an
  output format" since it auto-detects the container from the extension. Added
  -f mp4 to specify the format explicitly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Jellyfin.Plugin.CinemaTrailers4Jellyfins/Services/FakeMovieService.cs

@@ -97,13 +97,19 @@ namespace Jellyfin.Plugin.CinemaTrailers4Jellyfins.Services
                     "-shortest",
                     "-c:v libx264 -tune stillimage -pix_fmt yuv420p",
                     "-c:a aac -b:a 64k",
+                    "-f mp4",
                     $"\"{tempPath}\"");
 
+                // Prefer Jellyfin's bundled ffmpeg; fall back to whatever is on PATH.
+                var ffmpegPath = File.Exists("/usr/lib/jellyfin-ffmpeg/ffmpeg")
+                    ? "/usr/lib/jellyfin-ffmpeg/ffmpeg"
+                    : "ffmpeg";
+
                 using var process = new Process
                 {
                     StartInfo = new ProcessStartInfo
                     {
-                        FileName = "ffmpeg",
+                        FileName = ffmpegPath,
                         Arguments = args,
                         RedirectStandardOutput = true,
                         RedirectStandardError = true,
@@ -135,7 +141,7 @@ namespace Jellyfin.Plugin.CinemaTrailers4Jellyfins.Services
             }
             catch (Exception ex)
             {
-                _logger.LogError(ex, "|CinemaTrailers4Jellyfins| Failed to generate the master fake-movie file. Is ffmpeg installed and on PATH?");
+                _logger.LogError(ex, "|CinemaTrailers4Jellyfins| Failed to generate the master fake-movie file.");
                 return false;
             }
             finally