feat(sync): 聊天图片本地化(message media_hash+download_message_images+local-first rendering)

- Message 模型新增 media_hash 列 (sha256 FK -> Media.media_hash)
- Alembic 迁移 e5f6a7b8c9d0_message_media_hash (batch_alter_table, SQLite/MySQL 通用)
- app/scraper/media.py:download_message_images 批量下载器，镜像 download_call_recordings
  - 候选: media_hash IS NULL AND channel IN ('wechat','whatsapp') AND type IN ('image','video','file') AND 任一 file_*_url LIKE 'http%'
  - image 类型优先 file_th_url (缩略图，小快省)，video/file 优先 file_url
  - per-row try/except + per-run tried set，失败不挂循环
- sync_message_images orchestrator 入口 + ENTITY_MESSAGE_IMAGES 常量 (提交自身 SyncRun，镜像 call_recordings)
- scheduler: _run_message_images + ENTITY_FUNCS['message_images'] + 加入 daily all job (尾部)
- MessageOut schema 暴露 media_hash 字段
- MessageBubble.vue local-first: media_hash 优先 /api/media/{hash}，否则回退 OSS file_url/file_th_url/file_hd_url
- types.ts: Message 接口添加 media_hash 字段
- 新增 test_message_images_downloader.py (6 tests, 镜像 test_recording_downloader)

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

yunke-server/app/db/models.py

@@ -377,6 +377,14 @@ class Message(Base):
     timestamp: Mapped[int | None] = mapped_column(BigInteger, nullable=True)  # ms epoch
     is_del: Mapped[bool | None] = mapped_column(Boolean, nullable=True)
 
+    # sha256 media_hash of the localized media blob (FK -> Media.media_hash);
+    # NULL until sync_message_images downloads it. Populated by the
+    # message_images sync pass; rendering prefers /api/media/{this} over the
+    # OSS file_url / file_th_url / file_hd_url URLs. For image messages the
+    # localized blob is typically the thumbnail (file_th_url) — small + fast
+    # + sufficient for chat-timeline display.
+    media_hash: Mapped[str | None] = mapped_column(String(64), nullable=True)
+
     version: Mapped[str | None] = mapped_column(String(64), nullable=True)
     raw_json: Mapped[dict[str, Any] | None] = mapped_column(JSON, nullable=True)
     fetched_at: Mapped[dt.datetime] = _fetched_at()