Stop seeking into the video for poster generation, just use the first frame.

This guarantees that a poster can be generated for any valid stream instead of
breaking on extremely short streams (<1s).

pipeline/handlers/poster.py

@@ -17,7 +17,7 @@ def make_poster(inpath, outdir):
     outfile = f"{infile_noext}-poster.jpg"
     outpath = path.join(outdir, outfile)
 
-    ffmpeg.input(inpath, ss=1) \
+    ffmpeg.input(inpath) \
           .output(outpath, vframes=1) \
           .run(quiet=True)
     return (outdir, outfile)