Add debug logging for subtitle path and FFmpeg command

Escape subtitle path and add detailed logging to debug subtitle visibility issue.

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

app.py

@@ -513,20 +513,25 @@ def burn_subtitles_and_watermark(video_path: str, output_path: str, subtitle_pat
 
         # Step 2: Apply subtitles if needed (second pass)
         if subtitle_path and os.path.exists(subtitle_path):
-            print(f"[burn] Pass 2: Burning subtitles...")
+            print(f"[burn] Pass 2: Burning subtitles from {subtitle_path}...")
+
+            # Escape the subtitle path for FFmpeg (replace : with \\: and \ with /)
+            subtitle_path_escaped = subtitle_path.replace('\\', '/').replace(':', '\\:')
 
             cmd = [
                 'ffmpeg', '-y', '-i', current_video,
-                '-vf', f"subtitles={subtitle_path}:force_style='FontName=Montserrat SemiBold'",
+                '-vf', f"subtitles='{subtitle_path_escaped}':force_style='FontName=Montserrat SemiBold'",
                 '-c:a', 'copy', '-pix_fmt', 'yuv420p',
                 output_path
             ]
 
+            print(f"[burn] FFmpeg command: {' '.join(cmd)}")
             result = subprocess.run(cmd, capture_output=True, text=True)
             if result.returncode != 0:
-                raise Exception(f"Subtitle pass failed: {result.stderr[-200:]}")
+                print(f"[burn] Subtitle stderr: {result.stderr}")
+                raise Exception(f"Subtitle pass failed: {result.stderr[-500:]}")
 
-            print(f"[burn] Subtitles burned")
+            print(f"[burn] Subtitles burned successfully")
         elif current_video != video_path:
             # Only watermark was applied, move temp file to output
             import shutil
@@ -732,6 +737,10 @@ def full_generation_process(
         if add_subtitles and subtitle_segments:
             subtitle_file = tempfile.mktemp(suffix=".ass")
             create_beautiful_ass_subtitles(subtitle_segments, subtitle_file, int(width), int(height))
+            print(f"[subtitles] Created subtitle file: {subtitle_file}, exists: {os.path.exists(subtitle_file)}")
+            if os.path.exists(subtitle_file):
+                with open(subtitle_file, 'r') as f:
+                    print(f"[subtitles] File size: {len(f.read())} bytes")
 
         # Burn subtitles and/or watermark
         output_with_extras = tempfile.mktemp(suffix=".mp4")