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/src/alphapulse/news_research/api.py

@@ -3,7 +3,7 @@
 from datetime import UTC, datetime
 from typing import Annotated, Literal
 from fastapi import APIRouter, Depends, HTTPException, Query
-from pydantic import Field
+from pydantic import AwareDatetime, Field
 from sqlalchemy import select, update, func, or_, text
 from sqlalchemy.dialects.postgresql import insert
 from alphapulse.core.db import get_session
@@ -119,6 +119,15 @@ async def settings(session: DB):
     )
 
 
+@router.get("/coverage")
+async def coverage(session: DB, start_at: AwareDatetime | None = None):
+    row = await settings_row(session)
+    start = (start_at if start_at is not None else row.start_at).astimezone(UTC)
+    if start > datetime.now(UTC):
+        raise HTTPException(422, "回补起始时间不能晚于当前时间")
+    return success(await batches.coverage(session, start))
+
+
 @router.put("/settings")
 async def save(body: SettingsInput, session: DB):
     await settings_row(session)
@@ -147,9 +156,16 @@ async def pause(session: DB):
     return success(None)
 
 
+class EnableInput(Strict):
+    start_at: AwareDatetime
+
+
 @router.post("/enable")
-async def enable(session: DB):
+async def enable(session: DB, body: EnableInput | None = None):
     row = await settings_row(session)
+    start_at = (body.start_at if body is not None else row.start_at).astimezone(UTC)
+    if start_at > datetime.now(UTC):
+        raise HTTPException(422, "回补起始时间不能晚于当前时间")
     if not await session.scalar(text("SELECT pg_try_advisory_xact_lock(20261103, 2)")):
         raise HTTPException(409, "模型验证已在进行中")
     if row.enabled:
@@ -242,10 +258,12 @@ async def enable(session: DB):
     )
     if row.control_revision != control_revision or row.config != original_config:
         raise HTTPException(409, "验证期间已暂停或设置变化，请重新验证")
+    report = await batches.coverage(session, start_at)
+    row.start_at = start_at
     row.enabled = True
     row.control_revision += 1
     await session.commit()
-    return success(None)
+    return success(report)
 
 
 @router.post("/skill-templates")