fix: add [ ] escaping for FFmpeg drawtext & guard plan_config None

- thumbnail_generator: _overlay_title_on_image now escapes [ and ]
  characters to prevent FFmpeg filter syntax errors when title
  contains brackets (e.g. '[Tag]')
- render_adapter: use (plan_config or {}).get(...) to guard against
  None plan_config in cover candidate generation block

Addresses AI Code Review findings on PR #1365.

apps/worker/video_processing/render_adapter.py

@@ -584,7 +584,7 @@ class RenderAdapter:
             from video_processing.thumbnail_generator import extract_and_upload_cover_frames
 
             # 从 plan config 提取标题文字，叠加到封面候选帧上
-            _title_cfg = plan_config.get("title", {}) or {}
+            _title_cfg = (plan_config or {}).get("title", {}) or {}
             _title_text = (_title_cfg.get("text", "") or "").strip() if _title_cfg.get("enabled", True) else ""
 
             cover_candidates = extract_and_upload_cover_frames(

apps/worker/video_processing/thumbnail_generator.py

@@ -145,7 +145,7 @@ def _overlay_title_on_image(
 
     # 转义 drawtext 特殊字符
     # FFmpeg drawtext 需要转义: ' : % \ [ ]
-    escaped = title_text.replace("\\", "\\\\").replace("'", "’").replace(":", "\\:").replace("%", "%%")
+    escaped = title_text.replace("\\", "\\\\").replace("'", "’").replace(":", "\\:").replace("%", "%%").replace("[", "\\[").replace("]", "\\]")
     # 截断过长标题
     if len(escaped) > 60:
         escaped = escaped[:57] + "..."