Add configurable news research backfill and coverage reporting

Allow selecting a historical start time and checking unfinished news before enabling analysis. Report enrolled work outside the selected range and use the active server boundary while retaining paused drafts. Cover date validation, queue carryover, task completion and cross-tab responses.

backend/tests/test_news_research_workspace.py

@@ -260,6 +260,54 @@ def test_settings_contract_rejects_third_stage_and_unknown_efforts(trading):
         )
 
 
+def test_backfill_start_requires_an_explicit_timezone(trading):
+    from alphapulse.news_research.api import EnableInput
+
+    selected = EnableInput(start_at="2026-10-01T00:00:00+08:00")
+    assert selected.start_at == datetime(2026, 9, 30, 16, tzinfo=UTC)
+    with pytest.raises(ValueError, match="timezone_aware"):
+        EnableInput(start_at="2026-10-01T00:00:00")
+
+
+@pytest.mark.parametrize("endpoint", ["coverage", "enable"])
+def test_future_backfill_is_rejected_before_model_calls(trading, monkeypatch, endpoint):
+    import asyncio
+    from unittest.mock import AsyncMock
+    from fastapi import HTTPException
+    from alphapulse.news_research import api, service
+
+    future = datetime.now(UTC) + timedelta(days=1)
+    row = N(start_at=datetime.now(UTC), enabled=False)
+    session = N(get=AsyncMock(return_value=row), commit=AsyncMock())
+    config = AsyncMock(side_effect=AssertionError("不应调用模型"))
+    monkeypatch.setattr(service, "model_config", config)
+    call = (api.enable(session, api.EnableInput(start_at=future)) if endpoint == "enable"
+            else api.coverage(session, start_at=future))
+    with pytest.raises(HTTPException) as error:
+        asyncio.run(call)
+    assert error.value.status_code == 422
+    assert not row.enabled
+    config.assert_not_called()
+    session.commit.assert_not_called()
+
+
+def test_failed_enable_does_not_save_the_selected_backfill_start(trading, monkeypatch):
+    import asyncio
+    from unittest.mock import AsyncMock
+    from fastapi import HTTPException
+    from alphapulse.news_research import api, service
+
+    previous = datetime(2026, 10, 1, tzinfo=UTC)
+    row = N(start_at=previous, enabled=False, config={}, control_revision=7)
+    session = N(get=AsyncMock(return_value=row), scalar=AsyncMock(side_effect=[True, 0, 0]), commit=AsyncMock())
+    monkeypatch.setattr(service, "model_config", AsyncMock(side_effect=ValueError("Skill 不可用")))
+    with pytest.raises(HTTPException) as error:
+        asyncio.run(api.enable(session, api.EnableInput(start_at=previous - timedelta(days=30))))
+    assert error.value.status_code == 502
+    assert row.start_at == previous and not row.enabled and row.control_revision == 7
+    session.commit.assert_not_called()
+
+
 @pytest.mark.parametrize("tool", ["read_news", "read_package"])
 def test_compressed_members_read_frozen_positive_sources_and_reject_old_cache(
     trading, tool