feat(download): add video download capability for videos without official subtitles

- Add download_video() to BilibiliAdapter using yt-dlp
- Add bilibili_cookie config to Settings
- Update DownloadProcessor to call video download for bilibili videos
- Add idempotency check to ASRProcessor for existing subtitle files
- Fix indentation error in asr.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

kb_pipeline/processors/asr.py

@@ -115,6 +115,7 @@ class ASRProcessor:
         """执行 ASR 处理
 
         策略：
+        0. 先检查是否已有字幕文件（幂等性）
         1. 先尝试官方字幕（仅 B站）
         2. 无官方字幕则调用 Whisper
         3. 记录状态
@@ -131,13 +132,19 @@ class ASRProcessor:
         result = None
 
         try:
-            # Step 1: 尝试官方字幕（仅 B站）
-            if input_data.platform == "bilibili":
-                result = await self._try_official_subtitle(input_data)
+            # Step 0: 检查是否已有字幕文件（幂等性）
+            existing_result = self._check_existing_subtitle_files(input_data)
+            if existing_result:
+                logger.info(f"Found existing subtitle files, skipping processing")
+                result = existing_result
+            else:
+                # Step 1: 尝试官方字幕（仅 B站）
+                if input_data.platform == "bilibili":
+                    result = await self._try_official_subtitle(input_data)
 
-            # Step 2: 无官方字幕则调用 Whisper
-            if result is None:
-                result = await self._run_whisper(input_data)
+                # Step 2: 无官方字幕则调用 Whisper
+                if result is None:
+                    result = await self._run_whisper(input_data)
 
             # Step 3: 记录成功状态
             if self.state_manager: