feat: add full 4-stem demucs mode with MIDI instrument replacement

- Support mode='full' in handle_demucs_separating config
- Full mode: 4-stem separation → basic-pitch MIDI → fluidsynth render → mix
- Each instrument gets correct GM instrument (drums=channel10, bass=34, other=1)
- 2-stem mode preserved as default for backward compatibility
- Uses run_full.py standalone script on GPU server via SSH

app/ktv_adapter.py

@@ -156,9 +156,15 @@ async def handle_video_preparing(tenant_id, task_id, step_name, input_data, conf
 # ─── Demucs Separation ───────────────────────────────────────────────
 
 async def handle_demucs_separating(tenant_id, task_id, step_name, input_data, config):
-    """Run Demucs on GPU server to separate vocals and accompaniment."""
+    """Run Demucs on GPU server to separate vocals and accompaniment.
+    
+    Supports two modes via config.mode:
+      - "2stem" (default): classic vocals + no_vocals
+      - "full": 4-stem → MIDI replace instruments → mix accompaniment
+    """
     work_dir = _task_dir(task_id)
     gpu_dir = _gpu_task_dir(task_id)
+    mode = config.get("mode", "2stem")
 
     # Find audio path from deps
     audio_path = None
@@ -178,7 +184,18 @@ async def handle_demucs_separating(tenant_id, task_id, step_name, input_data, co
     remote_audio = f"{gpu_dir}/audio.mp3"
     await _copy_to_gpu(audio_path, remote_audio)
 
-    # Run Demucs on GPU
+    if mode == "full":
+        # ── 4-stem + MIDI replace pipeline ──────────────────────────
+        result = await _demucs_full_pipeline(task_id, remote_audio, gpu_dir, work_dir)
+    else:
+        # ── Classic 2-stem pipeline ─────────────────────────────────
+        result = await _demucs_2stem_pipeline(remote_audio, gpu_dir, work_dir)
+
+    return result
+
+
+async def _demucs_2stem_pipeline(remote_audio, gpu_dir, work_dir):
+    """Classic 2-stem vocal separation."""
     demucs_cmd = (
         f"cd {gpu_dir} && "
         f"source {GPU_DEMUCS_VENV}/bin/activate && "
@@ -189,7 +206,6 @@ async def handle_demucs_separating(tenant_id, task_id, step_name, input_data, co
     if rc != 0:
         raise ValueError(f"Demucs 分离失败: {stderr}")
 
-    # Copy results back
     vocals_local = os.path.join(work_dir, "vocals.wav")
     no_vocals_local = os.path.join(work_dir, "no_vocals.wav")
     base = os.path.splitext(os.path.basename(remote_audio))[0]
@@ -199,6 +215,60 @@ async def handle_demucs_separating(tenant_id, task_id, step_name, input_data, co
     return {
         "vocals_path": vocals_local,
         "no_vocals_path": no_vocals_local,
+        "mode": "2stem",
+    }
+
+
+async def _demucs_full_pipeline(task_id, remote_audio, gpu_dir, work_dir):
+    """4-stem separation → MIDI instrument replacement → accompaniment mix.
+    
+    Runs the standalone run_full.py on GPU server via SSH.
+    """
+    import json as _json
+
+    output_dir = f"{gpu_dir}/full_output"
+
+    cmd = (
+        f"cd /data/ymq/demucs-service && "
+        f"DEMUCS_GPU_ID=5 "
+        f"{GPU_DEMUCS_VENV}/bin/python run_full.py "
+        f"'{remote_audio}' '{output_dir}'"
+    )
+
+    logger.info(f"[demucs-full] Starting full pipeline for task {task_id}")
+    stdout, stderr, rc = await _run_gpu(cmd, timeout=900)
+
+    if rc != 0:
+        raise ValueError(f"Demucs full separation failed: {stderr[-500:]}")
+
+    # Parse JSON result from stdout
+    try:
+        gpu_result = _json.loads(stdout)
+    except _json.JSONDecodeError:
+        # Try to extract JSON from mixed output
+        lines = stdout.strip().split("\n")
+        for line in reversed(lines):
+            try:
+                gpu_result = _json.loads(line)
+                break
+            except _json.JSONDecodeError:
+                continue
+        else:
+            raise ValueError(f"Failed to parse demucs output: {stdout[-500:]}")
+
+    # Copy results back to local
+    vocals_local = os.path.join(work_dir, "vocals.wav")
+    accompaniment_local = os.path.join(work_dir, "accompaniment.wav")
+
+    await _copy_from_gpu(gpu_result["vocals_path"], vocals_local)
+    await _copy_from_gpu(gpu_result["accompaniment_path"], accompaniment_local)
+
+    return {
+        "vocals_path": vocals_local,
+        "no_vocals_path": accompaniment_local,     # backward compat
+        "accompaniment_path": accompaniment_local,  # new name
+        "mode": "full",
+        "duration": gpu_result.get("duration", 0),
     }