m4b bug fix

app.py

@@ -159,14 +159,46 @@ def parse_uploaded_file(file_path):
     return None, None
 
 async def convert_to_m4b(mp3_path, output_filename):
-    """Convert MP3 to M4B format using pydub"""
+    """Convert MP3 to M4B format using ffmpeg directly (supports large files)"""
     try:
-        audio = AudioSegment.from_mp3(mp3_path)
+        import subprocess
+        
         m4b_path = tempfile.NamedTemporaryFile(delete=False, suffix=".m4b").name
-        # Export as m4a (m4b is essentially m4a with audiobook metadata)
-        audio.export(m4b_path, format="ipod", codec="aac")
+        
+        # Use ffmpeg directly for conversion (avoids pydub's 4GB limit)
+        cmd = [
+            'ffmpeg', '-y',  # Overwrite output
+            '-i', mp3_path,  # Input file
+            '-c:a', 'aac',   # Audio codec
+            '-b:a', '128k',  # Audio bitrate
+            '-f', 'ipod',    # M4B/M4A format
+            m4b_path
+        ]
+        
+        logger.info(f"Running ffmpeg conversion: {' '.join(cmd)}")
+        
+        result = subprocess.run(
+            cmd,
+            capture_output=True,
+            text=True,
+            timeout=3600  # 1 hour timeout for large files
+        )
+        
+        if result.returncode != 0:
+            logger.error(f"ffmpeg error: {result.stderr}")
+            if os.path.exists(m4b_path):
+                os.remove(m4b_path)
+            return None
+        
         logger.info(f"Converted to M4B: {m4b_path}")
         return m4b_path
+        
+    except FileNotFoundError:
+        logger.error("ffmpeg not found. Please install ffmpeg to use M4B format.")
+        return None
+    except subprocess.TimeoutExpired:
+        logger.error("ffmpeg conversion timed out")
+        return None
     except Exception as e:
         logger.error(f"Failed to convert to M4B: {e}")
         return None