Use ZeroGPU for stem separation in song package

- Update separate_stems to support two_stem parameter for 2-pass separation
- Song package now uses GPU-accelerated Demucs for both passes
- Pass 1: vocals/instrumental (2-stem mode)
- Pass 2: drums, bass, guitar, keys, other, vocals (6-stem mode)
- Proper quota error handling for both passes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app.py

@@ -400,18 +400,26 @@ def download_youtube(url: str) -> tuple:
         return None, str(e)
 
 @spaces.GPU(duration=120)
-def _separate_stems_gpu(audio_path: str, progress=None) -> dict:
+def _separate_stems_gpu(audio_path: str, progress=None, two_stem: bool = False) -> dict:
     """Separate audio into stems using Demucs (GPU accelerated) - internal"""
     try:
         import subprocess
-        stems_dir = OUTPUT_DIR / f"stems_{datetime.now().strftime('%H%M%S')}"
+        mode = "2stem" if two_stem else "6stem"
+        stems_dir = OUTPUT_DIR / f"stems_{mode}_{datetime.now().strftime('%H%M%S')}"
         stems_dir.mkdir(exist_ok=True)
 
         if progress:
-            progress(0.3, "Running Demucs stem separation...")
+            if two_stem:
+                progress(0.3, "Running Demucs 2-stem separation (vocals/instrumental)...")
+            else:
+                progress(0.3, "Running Demucs 6-stem separation (drums, bass, guitar, keys, other, vocals)...")
+
+        # Build command based on mode
+        if two_stem:
+            cmd = ['python', '-m', 'demucs', '--two-stems=vocals', '-o', str(stems_dir), '--mp3', '--mp3-bitrate=320', audio_path]
+        else:
+            cmd = ['python', '-m', 'demucs', '-n', 'htdemucs_6s', '-o', str(stems_dir), '--mp3', '--mp3-bitrate=320', audio_path]
 
-        # Use GPU if available
-        cmd = ['python', '-m', 'demucs', '-o', str(stems_dir), audio_path]
         subprocess.run(cmd, capture_output=True, check=True)
 
         # Find output stems
@@ -419,18 +427,24 @@ def _separate_stems_gpu(audio_path: str, progress=None) -> dict:
         song_dir = list(model_dir.glob('*'))[0]
 
         stems = {}
-        for stem_file in song_dir.glob('*.wav'):
+        for stem_file in song_dir.glob('*.mp3'):
             stem_name = stem_file.stem
             stems[stem_name] = str(stem_file)
 
-        return stems
+        # Also check for wav files
+        for stem_file in song_dir.glob('*.wav'):
+            stem_name = stem_file.stem
+            if stem_name not in stems:
+                stems[stem_name] = str(stem_file)
+
+        return {'stems': stems, 'stems_dir': str(song_dir)}
     except Exception as e:
         return {'error': str(e)}
 
-def separate_stems(audio_path: str, progress=None) -> dict:
+def separate_stems(audio_path: str, progress=None, two_stem: bool = False) -> dict:
     """Separate stems - wrapper with quota error handling"""
     try:
-        return _separate_stems_gpu(audio_path, progress)
+        return _separate_stems_gpu(audio_path, progress, two_stem)
     except Exception as e:
         return {'error': handle_gpu_error(e)}
 
@@ -619,39 +633,45 @@ def create_song_package(audio, yt_url, name, lyrics, user_email, progress=gr.Pro
     key = analysis.get('key', 'C')
     log.append(f"Key: {key} | BPM: {tempo}")
 
-    # === PASS 1: 2-STEM SEPARATION (vocals/instrumental) ===
-    stems_pass1_dir = package_dir / "stems_pass1"
+    # === PASS 1: 2-STEM SEPARATION (vocals/instrumental) - GPU ACCELERATED ===
     stems_pass1 = {}
+    stems_pass1_dir = None
 
-    progress(0.15, "Pass 1: Separating vocals/instrumental...")
+    progress(0.15, "Pass 1: Separating vocals/instrumental (GPU)...")
     try:
-        if HAS_STEM_MODULE:
-            stems_pass1_output = module_separate_stems(audio_path, str(stems_pass1_dir), two_stem=True)
-            if stems_pass1_output:
-                for stem_file in Path(stems_pass1_output).glob('*.mp3'):
-                    stems_pass1[stem_file.stem] = str(stem_file)
-                log.append(f"Pass 1 stems: {', '.join(stems_pass1.keys())}")
+        result = separate_stems(audio_path, progress, two_stem=True)
+        if 'error' in result:
+            error_msg = result['error']
+            if is_quota_error(str(error_msg)):
+                log.append(QUOTA_ERROR_MSG)
+            else:
+                log.append(f"Pass 1 error: {error_msg}")
         else:
-            log.append("Pass 1: Stem module not available")
+            stems_pass1 = result.get('stems', {})
+            stems_pass1_dir = result.get('stems_dir')
+            log.append(f"Pass 1 stems: {', '.join(stems_pass1.keys())}")
     except Exception as e:
-        log.append(f"Pass 1 error: {str(e)}")
+        log.append(f"Pass 1 error: {handle_gpu_error(e)}")
 
-    # === PASS 2: 6-STEM SEPARATION (detailed) ===
-    stems_pass2_dir = package_dir / "stems_pass2"
+    # === PASS 2: 6-STEM SEPARATION (detailed) - GPU ACCELERATED ===
     stems_pass2 = {}
+    stems_pass2_dir = None
 
-    progress(0.35, "Pass 2: Separating detailed stems (drums, bass, guitar, keys, other)...")
+    progress(0.35, "Pass 2: Separating detailed stems (GPU)...")
     try:
-        if HAS_STEM_MODULE:
-            stems_pass2_output = module_separate_stems(audio_path, str(stems_pass2_dir), two_stem=False)
-            if stems_pass2_output:
-                for stem_file in Path(stems_pass2_output).glob('*.mp3'):
-                    stems_pass2[stem_file.stem] = str(stem_file)
-                log.append(f"Pass 2 stems: {', '.join(stems_pass2.keys())}")
+        result = separate_stems(audio_path, progress, two_stem=False)
+        if 'error' in result:
+            error_msg = result['error']
+            if is_quota_error(str(error_msg)):
+                log.append(QUOTA_ERROR_MSG)
+            else:
+                log.append(f"Pass 2 error: {error_msg}")
         else:
-            log.append("Pass 2: Stem module not available")
+            stems_pass2 = result.get('stems', {})
+            stems_pass2_dir = result.get('stems_dir')
+            log.append(f"Pass 2 stems: {', '.join(stems_pass2.keys())}")
     except Exception as e:
-        log.append(f"Pass 2 error: {str(e)}")
+        log.append(f"Pass 2 error: {handle_gpu_error(e)}")
 
     # === MASTER ORIGINAL (Warm preset) ===
     mastered_path = None
@@ -711,14 +731,9 @@ def create_song_package(audio, yt_url, name, lyrics, user_email, progress=gr.Pro
     progress(0.80, "Creating Reaper project...")
     try:
         if HAS_REAPER_MODULE:
-            # Use Pass 2 stems for Reaper (more detailed)
-            stems_for_reaper = stems_pass2_dir if stems_pass2 else (stems_pass1_dir if stems_pass1 else None)
+            # Use Pass 2 stems for Reaper (more detailed), fall back to Pass 1
+            stems_for_reaper = stems_pass2_dir if stems_pass2 else stems_pass1_dir
             if stems_for_reaper and Path(stems_for_reaper).exists():
-                # Find the actual stems directory (Demucs creates nested dirs)
-                possible_dirs = list(Path(stems_for_reaper).glob('*/*'))
-                if possible_dirs:
-                    stems_for_reaper = possible_dirs[0]
-
                 rpp_content = create_reaper_project(
                     song_name=song_name,
                     stems_dir=stems_for_reaper,