feat(bilibili): 优先走官方 player API 直拉字幕

之前 BilibiliDownloader.download_subtitles 走的是 yt-dlp 的 writesubtitles 路径，对 B 站签名/Cookie 的兼容性差，常常空手而归，落到音频下载 + Whisper 转写的慢路径。

新增 bilibili_subtitle.BilibiliSubtitleFetcher：
- /x/web-interface/view?bvid=... → 拿 cid
- /x/player/wbi/v2?bvid=...&cid=... → 拿 subtitle 列表（subtitle_url 已带 auth_key）
- 优先级：人工中文 > AI 中文 > 任意中文 > 任意非空
- fetch JSON body 解析为 TranscriptResult
- 通过 CookieConfigManager 自动注入 SESSDATA cookie（AI 字幕必需）

bilibili_downloader.download_subtitles 顺序改为：先试新 fetcher，失败再回退到原 yt-dlp 路径。NoteGenerator 的字幕优先逻辑无需改动——它本来就调 download_subtitles。

效果：
- B 站视频如果有字幕（人工或 AI），直接秒拿，跳过音频下载 + 转写
- 完全绕开 MLX Whisper 不可用 / 模型未下载 等转写器问题
- 拿不到字幕时仍可走原音频转写路径

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/downloaders/bilibili_downloader.py

@@ -8,6 +8,7 @@ from typing import Union, Optional, List
 import yt_dlp
 
 from app.downloaders.base import Downloader, DownloadQuality, QUALITY_MAP
+from app.downloaders.bilibili_subtitle import BilibiliSubtitleFetcher
 from app.models.notes_model import AudioDownloadResult
 from app.models.transcriber_model import TranscriptResult, TranscriptSegment
 from app.utils.path_helper import get_data_dir
@@ -155,6 +156,15 @@ class BilibiliDownloader(Downloader, ABC):
         :param langs: 优先语言列表
         :return: TranscriptResult 或 None
         """
+        # 1) 优先走 B 站官方 player API（直拉，无需下视频；AI 字幕需 SESSDATA cookie）
+        try:
+            result = BilibiliSubtitleFetcher().fetch_subtitles(video_url)
+            if result and result.segments:
+                return result
+        except Exception as e:
+            logger.warning(f"player API 直拉字幕异常，回退到 yt-dlp: {e}")
+
+        # 2) Fallback：原 yt-dlp 路径（更脆弱，遇到签名/Cookie 问题失败概率较高）
         if output_dir is None:
             output_dir = get_data_dir()
         if not output_dir:

backend/app/downloaders/bilibili_subtitle.py

@@ -0,0 +1,164 @@
+"""
+直接调用 B 站 player API 拿字幕，绕过 yt-dlp。
+
+流程：
+1. 从 URL 提 BV id（已有 utils.url_parser.extract_video_id）
+2. GET /x/web-interface/view?bvid=BVxxx → 拿 cid
+3. GET /x/player/wbi/v2?bvid=...&cid=... → 返回 data.subtitle.subtitles[]
+   每条带 subtitle_url（B 站后端已经签好 auth_key 的完整地址）
+4. 按优先级（人工 zh-CN > AI zh-CN > 任意 zh > 任意非空）选一条
+5. fetch subtitle_url → JSON {body:[{from,to,content,...}]}
+6. 解析为 TranscriptResult
+
+AI 字幕需要登录态 cookie（SESSDATA）；通过 CookieConfigManager 注入。
+"""
+
+from typing import List, Optional
+
+import requests
+
+from app.models.transcriber_model import TranscriptResult, TranscriptSegment
+from app.services.cookie_manager import CookieConfigManager
+from app.utils.logger import get_logger
+from app.utils.url_parser import extract_video_id
+
+logger = get_logger(__name__)
+
+UA = (
+    "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) "
+    "AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36"
+)
+
+
+class BilibiliSubtitleFetcher:
+    """通过 B 站官方 API 直拉字幕。"""
+
+    def __init__(self):
+        self._cookie = CookieConfigManager().get("bilibili") or ""
+
+    def _headers(self) -> dict:
+        h = {
+            "User-Agent": UA,
+            "Referer": "https://www.bilibili.com",
+        }
+        if self._cookie:
+            h["Cookie"] = self._cookie
+        return h
+
+    def _get_cid(self, bvid: str) -> Optional[int]:
+        url = "https://api.bilibili.com/x/web-interface/view"
+        try:
+            resp = requests.get(url, params={"bvid": bvid}, headers=self._headers(), timeout=10)
+            data = resp.json()
+        except Exception as e:
+            logger.warning(f"获取 cid 失败: {e}")
+            return None
+        if data.get("code") != 0:
+            logger.warning(f"view API 返回错误: code={data.get('code')}, msg={data.get('message')}")
+            return None
+        cid = data.get("data", {}).get("cid")
+        return int(cid) if cid else None
+
+    def _list_subtitles(self, bvid: str, cid: int) -> List[dict]:
+        url = "https://api.bilibili.com/x/player/wbi/v2"
+        try:
+            resp = requests.get(url, params={"bvid": bvid, "cid": cid}, headers=self._headers(), timeout=10)
+            data = resp.json()
+        except Exception as e:
+            logger.warning(f"获取字幕列表失败: {e}")
+            return []
+        if data.get("code") != 0:
+            logger.warning(f"player API 返回错误: code={data.get('code')}, msg={data.get('message')}")
+            return []
+        subtitles = data.get("data", {}).get("subtitle", {}).get("subtitles", [])
+        return subtitles or []
+
+    def _pick(self, subtitles: List[dict]) -> Optional[dict]:
+        """优先级：人工中文 > AI 中文 > 任意中文 > 任意非空。"""
+        if not subtitles:
+            return None
+
+        def is_zh(s: dict) -> bool:
+            lan = (s.get("lan") or "").lower()
+            return lan.startswith("zh") or lan == "ai-zh"
+
+        # 人工中文（type 0=AI, 1=人工 ；ai_type=0 视为人工）
+        for s in subtitles:
+            if is_zh(s) and not s.get("ai_type"):
+                return s
+        # AI 中文
+        for s in subtitles:
+            if is_zh(s):
+                return s
+        # 任意非空
+        return subtitles[0]
+
+    @staticmethod
+    def _normalize_url(url: str) -> str:
+        if url.startswith("//"):
+            return "https:" + url
+        return url
+
+    def _fetch_body(self, subtitle_url: str) -> Optional[List[dict]]:
+        try:
+            resp = requests.get(self._normalize_url(subtitle_url), headers=self._headers(), timeout=15)
+            data = resp.json()
+            return data.get("body") or []
+        except Exception as e:
+            logger.warning(f"下载字幕 JSON 失败: {e}")
+            return None
+
+    def fetch_subtitles(self, video_url: str) -> Optional[TranscriptResult]:
+        bvid = extract_video_id(video_url, "bilibili")
+        if not bvid:
+            logger.info("无法从 URL 提取 BV id")
+            return None
+
+        cid = self._get_cid(bvid)
+        if not cid:
+            logger.info(f"{bvid} 没有取到 cid")
+            return None
+
+        subtitles = self._list_subtitles(bvid, cid)
+        if not subtitles:
+            logger.info(f"{bvid} (cid={cid}) 没有可用字幕轨")
+            return None
+
+        track = self._pick(subtitles)
+        if not track or not track.get("subtitle_url"):
+            logger.info(f"{bvid} 字幕轨存在但没有 subtitle_url（可能未登录、需要 SESSDATA cookie）")
+            return None
+
+        lan = track.get("lan") or "zh"
+        body = self._fetch_body(track["subtitle_url"])
+        if not body:
+            return None
+
+        segments: List[TranscriptSegment] = []
+        for item in body:
+            text = (item.get("content") or "").strip()
+            if not text:
+                continue
+            segments.append(TranscriptSegment(
+                start=float(item.get("from", 0)),
+                end=float(item.get("to", 0)),
+                text=text,
+            ))
+
+        if not segments:
+            return None
+
+        full_text = " ".join(s.text for s in segments)
+        logger.info(f"B站直拉字幕成功: {bvid} lan={lan} 共 {len(segments)} 段")
+        return TranscriptResult(
+            language=lan,
+            full_text=full_text,
+            segments=segments,
+            raw={
+                "source": "bilibili_player_api",
+                "bvid": bvid,
+                "cid": cid,
+                "lan": lan,
+                "ai_type": track.get("ai_type"),
+            },
+        )