feat: LLM Batch API — OpenAI + Anthropic 50% cost reduction (v0.9.8)

Submit up to 1000 unbriefed documents to the provider Batch API in one
shot instead of individual synchronous LLM calls. Results are polled
every 30 minutes via a new Celery beat task and imported automatically.

- New worker: llm_batch_processor.py
  - submit_llm_batch: guards against duplicate batches, builds JSONL
    (OpenAI) or request list (Anthropic), stores state in AppSetting
  - poll_llm_batch_results: checks batch status, imports completed
    results with idempotency, emits notifications + triggers news fetch
- celery_app: register worker, route to llm queue, beat every 30 min
- admin API: POST /submit-llm-batch + GET /llm-batch-status endpoints
- Frontend: submitLlmBatch + getLlmBatchStatus in adminAPI; settings
  page shows batch control row (openai/anthropic only) with live
  progress line while batch is processing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/api/admin.py

@@ -281,6 +281,31 @@ async def clear_gnews_cache_endpoint(current_user: User = Depends(get_current_ad
     return {"cleared": cleared}
 
 
+@router.post("/submit-llm-batch")
+async def submit_llm_batch_endpoint(current_user: User = Depends(get_current_admin)):
+    """Submit all unbriefed documents to the Batch API (OpenAI/Anthropic only)."""
+    from app.workers.llm_batch_processor import submit_llm_batch
+    task = submit_llm_batch.delay()
+    return {"task_id": task.id, "status": "queued"}
+
+
+@router.get("/llm-batch-status")
+async def get_llm_batch_status(
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_admin),
+):
+    """Return the current batch job state, or no_active_batch if none."""
+    import json
+    from app.models.setting import AppSetting
+    row = await db.get(AppSetting, "llm_active_batch")
+    if not row:
+        return {"status": "no_active_batch"}
+    try:
+        return json.loads(row.value)
+    except Exception:
+        return {"status": "unknown"}
+
+
 @router.get("/api-health")
 async def api_health(current_user: User = Depends(get_current_admin)):
     """Test each external API and return status + latency for each."""