修复线上视频导出失败 + 配音预览音画同步

- Dockerfile: 补装 ffmpeg 与中文字体 fonts-noto-cjk,修线上导出报
  "No such file or directory: 'ffmpeg'" 及字幕中文方块;换阿里云 apt 源防构建超时
- export.py: 字幕字体加载加 Noto CJK glob 兜底,适配不同版本 fonts-noto-cjk
- pipeline.tsx: 配音预览改 Web Audio 精确排程(借鉴 WebAV)——预解码 +
  source.start(when,offset) + 增量重排,正在发声的句绝不掐断,消除字幕音错位与卡顿

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/backend/apps/projects/services/export.py

@@ -65,6 +65,20 @@ def _load_font(size: int):
             return ImageFont.truetype(path, size, index=0)
         except Exception:  # noqa: BLE001
             continue
+    # 兜底:扫常见字体目录里任意 Noto CJK 文件(fonts-noto-cjk 不同版本文件名可能不同),
+    # 找不到才退英文位图字体(中文会变方块)——保证线上装了 noto-cjk 就一定能用上
+    import glob
+
+    for pattern in (
+        "/usr/share/fonts/**/NotoSansCJK*.ttc",
+        "/usr/share/fonts/**/NotoSansCJK*.otf",
+        "/usr/share/fonts/**/NotoSerifCJK*.ttc",
+    ):
+        for path in glob.glob(pattern, recursive=True):
+            try:
+                return ImageFont.truetype(path, size, index=0)
+            except Exception:  # noqa: BLE001
+                continue
     return ImageFont.load_default()