fix: 字幕居中bug修复，BGM声音太小bug修复，默认system prompt微调 - 强化旁白字数控制

- composer.py: BGM音量调整为0.45，禁用ducking压缩
- ffmpeg_utils.py: 字幕居中修复，多行字幕每行单独居中
- script_gen.py: 清理调试代码

modules/ffmpeg_utils.py

@@ -524,29 +524,44 @@ def add_multiple_subtitles(
         fontcolor = style.get("fontcolor", "white")
         borderw = style.get("borderw", 3)
         bordercolor = style.get("bordercolor", "black")
-        x = style.get("x", "(w-text_w)/2")
-        y = style.get("y", "h-200")
+        base_y = style.get("y", "h-200")
         
         # 默认启用背景框以提高可读性
         box = style.get("box", 1)
         boxcolor = style.get("boxcolor", "black@0.5")
         boxborderw = style.get("boxborderw", 10)
         
-        # 转义：反斜杠、单引号、冒号、百分号
-        escaped_text = text.replace("\\", "\\\\").replace("'", "\\'").replace(":", "\\:").replace("%", "\\%")
+        # 多行字幕：拆分成多个 drawtext 滤镜，每行单独居中
+        lines = text.split("\n") if "\n" in text else [text]
+        line_height = int(fontsize * 1.3)  # 行高
         
-        drawtext = (
-            f"drawtext=text='{escaped_text}':"
-            f"fontfile='{font}':"
-            f"fontsize={fontsize}:"
-            f"fontcolor={fontcolor}:"
-            f"borderw={borderw}:"
-            f"bordercolor={bordercolor}:"
-            f"box={box}:boxcolor={boxcolor}:boxborderw={boxborderw}:"
-            f"x={x}:y={y}:"
-            f"enable='between(t,{start},{start + duration})'"
-        )
-        filters.append(drawtext)
+        for line_idx, line in enumerate(lines):
+            if not line.strip():
+                continue
+                
+            # 转义：反斜杠、单引号、冒号、百分号
+            escaped_line = line.replace("\\", "\\\\").replace("'", "\\'").replace(":", "\\:").replace("%", "\\%")
+            
+            # 计算每行的 y 位置（从底部往上排列）
+            # base_y 是最后一行的位置，往上依次排列
+            line_offset = (len(lines) - 1 - line_idx) * line_height
+            if isinstance(base_y, str) and base_y.startswith("h-"):
+                y_expr = f"({base_y})-{line_offset}"
+            else:
+                y_expr = f"({base_y})-{line_offset}"
+            
+            drawtext = (
+                f"drawtext=text='{escaped_line}':"
+                f"fontfile='{font}':"
+                f"fontsize={fontsize}:"
+                f"fontcolor={fontcolor}:"
+                f"borderw={borderw}:"
+                f"bordercolor={bordercolor}:"
+                f"box={box}:boxcolor={boxcolor}:boxborderw={boxborderw}:"
+                f"x=(w-text_w)/2:y={y_expr}:"  # 每行都水平居中
+                f"enable='between(t,{start},{start + duration})'"
+            )
+            filters.append(drawtext)
     
     # 用逗号连接多个 filter
     vf = ",".join(filters)
@@ -809,6 +824,16 @@ def add_bgm(
         bgm_volume: BGM音量
         loop: 是否循环BGM
     """
+    # 验证 BGM 文件存在
+    if not bgm_path or not os.path.exists(bgm_path):
+        logger.error(f"BGM file not found: {bgm_path}")
+        # 直接复制原视频，不添加 BGM
+        import shutil
+        shutil.copy(video_path, output_path)
+        return output_path
+    
+    logger.info(f"Adding BGM: {bgm_path} (volume={bgm_volume})")
+    
     info = get_video_info(video_path)
     video_duration = info["duration"]