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/alembic/versions/20261013_d41b7c2e9a10_source_history.py

@@ -0,0 +1,32 @@
+"""source history backfill
+
+Revision ID: d41b7c2e9a10
+Revises: bf0ffbfcaf9a
+Create Date: 2026-10-13 15:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'd41b7c2e9a10'
+down_revision: Union[str, Sequence[str], None] = 'bf0ffbfcaf9a'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    op.add_column('sources', sa.Column('history_target', sa.DateTime(timezone=True), nullable=True))
+    op.add_column('sources', sa.Column('history_position', sa.DateTime(timezone=True), nullable=True))
+    op.add_column('sources', sa.Column('history_done', sa.Boolean(), server_default='false', nullable=False))
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.drop_column('sources', 'history_done')
+    op.drop_column('sources', 'history_position')
+    op.drop_column('sources', 'history_target')