Fix PNG watermark alpha channel handling

Name filter output and explicitly map it to ensure alpha transparency is properly respected.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app.py

@@ -501,17 +501,17 @@ def burn_subtitles_and_watermark(video_path: str, output_path: str, subtitle_pat
             filter_complex = (
                 f"[0:v]subtitles={subtitle_path}:force_style='FontName=Montserrat SemiBold'[v];"
                 f"[1:v][v]scale2ref[ovr][base];"
-                f"[base][ovr]overlay=0:0"
+                f"[base][ovr]overlay=0:0[vout]"
             )
-            cmd.extend(['-filter_complex', filter_complex])
+            cmd.extend(['-filter_complex', filter_complex, '-map', '[vout]', '-map', '0:a?'])
         elif subtitle_path and os.path.exists(subtitle_path):
             # Only subtitles
             filter_complex = f"subtitles={subtitle_path}:force_style='FontName=Montserrat SemiBold'"
             cmd.extend(['-vf', filter_complex])
         elif watermark_path and os.path.exists(watermark_path):
-            # Only watermark: use scale2ref to properly handle PNG alpha channel
-            filter_complex = "[1:v][0:v]scale2ref[ovr][base];[base][ovr]overlay=0:0"
-            cmd.extend(['-filter_complex', filter_complex])
+            # Only watermark: use scale2ref to properly handle PNG alpha channel, name output and explicitly map it
+            filter_complex = "[1:v][0:v]scale2ref[ovr][base];[base][ovr]overlay=0:0[vout]"
+            cmd.extend(['-filter_complex', filter_complex, '-map', '[vout]', '-map', '0:a?'])
 
         cmd.extend(['-c:a', 'copy', '-pix_fmt', 'yuv420p', output_path])