Add resumable news history backfill

Let the crawler console set a backfill target per source. Each scheduled
run pages backwards from a checkpoint for at most 20s so real-time
crawling is not blocked. The checkpoint is saved only after a successful
run and is also kept in process memory, because the next trigger may be
queued before the database write lands. cls_depth is excluded since its
list API cannot page into history.

backend/src/alphapulse/models/news.py

@@ -21,6 +21,10 @@ class Source(Base):
     last_success_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     last_error: Mapped[str | None] = mapped_column(Text)
     paused_until: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    # 历史回补：目标时间（为空表示不回补）、已回补到的最早时间（断点）、是否已结束（到达目标或来源无更早数据）
+    history_target: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    history_position: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
+    history_done: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
 
 
 class NewsItem(Base):