Replace stems URL input with radio buttons + add download button

- Radio buttons to select Version A or B MP3 as stems source
- New /api/audio-source endpoint serves single MP3 for Extraction API
- Backend reads radio selection, constructs public URL automatically
- Download Stems button appears when stems are ready
- Generate button disabled when no MP3s are available

app.py

@@ -714,6 +714,73 @@ def stream_audio(token, version):
     return response
 
 
+@app.route('/api/audio-source/<token>/<version>.mp3')
+def audio_source(token, version):
+    """
+    Serve a single MP3 file for stems extraction.
+
+    Unlike /api/stream (which requires both A and B to exist for the customer
+    player), this endpoint serves whichever version is requested, as long as
+    that file exists. This URL is given to the MusicGPT Extraction API so it
+    can download the source audio for stem separation.
+    """
+    req = get_request_by_token(token)
+    if not req:
+        abort(404)
+    if version not in ('a', 'b'):
+        abort(404)
+    path = req.get('song_a_path') if version == 'a' else req.get('song_b_path')
+    if not path or not Path(path).exists():
+        abort(404)
+
+    file_path = Path(path)
+    file_size = file_path.stat().st_size
+
+    range_header = request.headers.get('Range', '')
+    start = 0
+    end = file_size - 1
+    status_code = 200
+
+    if range_header and range_header.startswith('bytes='):
+        try:
+            range_value = range_header[len('bytes='):].strip()
+            if '-' in range_value:
+                parts = range_value.split('-')
+                if parts[0]:
+                    start = int(parts[0])
+                if parts[1]:
+                    end = min(int(parts[1]), file_size - 1)
+            if start >= file_size or start > end:
+                abort(416)
+            status_code = 206
+        except ValueError:
+            start = 0
+            end = file_size - 1
+            status_code = 200
+
+    def generate():
+        with open(file_path, 'rb') as f:
+            f.seek(start)
+            remaining = end - start + 1
+            chunk_size = 64 * 1024
+            while remaining > 0:
+                to_read = min(chunk_size, remaining)
+                data = f.read(to_read)
+                if not data:
+                    break
+                yield data
+                remaining -= len(data)
+
+    response = current_app.response_class(generate(), mimetype='audio/mpeg')
+    response.status_code = status_code
+    response.headers['Accept-Ranges'] = 'bytes'
+    response.headers['Content-Disposition'] = 'inline'
+    response.headers['Content-Length'] = str(end - start + 1)
+    if status_code == 206:
+        response.headers['Content-Range'] = f'bytes {start}-{end}/{file_size}'
+    return response
+
+
 @app.route('/audio/<token>/<version>.mp3')
 def audio(token, version):
     """
@@ -1053,7 +1120,7 @@ def admin_request(rid):
             return redirect(url_for('admin_request', rid=rid))
 
         elif action == 'generate_stems':
-            # Queue a stem extraction job for Version A MP3.
+            # Queue a stem extraction job for the selected MP3 (Version A or B).
             api_key = get_musicgpt_api_key()
             if not api_key:
                 flash('MusicGPT API key is not configured.', 'error')
@@ -1061,16 +1128,20 @@ def admin_request(rid):
             if req.get('stems_status') in ('IN_QUEUE', 'IN_PROGRESS'):
                 flash('A stems extraction is already in progress.', 'error')
                 return redirect(url_for('admin_request', rid=rid))
-            audio_url = request.form.get('stems_source_url', '').strip()
-            if not audio_url:
-                # Fall back to using the locally stored Version A MP3 path; upload not supported by extraction API.
-                a_path = req.get('song_a_path')
-                if a_path:
-                    # Extraction API supports audio_url; we need a public URL. Local path won't work.
-                    flash('Please provide a public audio URL for stems extraction.', 'error')
-                    return redirect(url_for('admin_request', rid=rid))
-                flash('No Version A audio available to extract stems from.', 'error')
+            # Operator selects which version (A or B) to extract stems from.
+            selected = request.form.get('stems_source', '').strip()
+            if selected == 'a':
+                song_path = req.get('song_a_path')
+            elif selected == 'b':
+                song_path = req.get('song_b_path')
+            else:
+                flash('Please select Version A or Version B for stems extraction.', 'error')
                 return redirect(url_for('admin_request', rid=rid))
+            if not song_path or not Path(song_path).exists():
+                flash(f'Version {selected.upper()} MP3 is not available. Generate or upload it first.', 'error')
+                return redirect(url_for('admin_request', rid=rid))
+            # Build a public URL the MusicGPT Extraction API can fetch.
+            audio_url = f"{current_app.config['PUBLIC_BASE_URL'].rstrip('/')}/api/audio-source/{req['player_token']}/{selected}.mp3"
             stems = request.form.getlist('stems') or ['vocals', 'instrumental']
             task_id, conv_id, estimate, error = musicgpt_queue_stems(rid, audio_url, stems=stems)
             if error: