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/workers/celery_app.py

@@ -16,6 +16,7 @@ celery_app = Celery(
         "app.workers.trend_scorer",
         "app.workers.member_interest",
         "app.workers.notification_dispatcher",
+        "app.workers.llm_batch_processor",
     ],
 )
 
@@ -35,6 +36,7 @@ celery_app.conf.update(
         "app.workers.congress_poller.*": {"queue": "polling"},
         "app.workers.document_fetcher.*": {"queue": "documents"},
         "app.workers.llm_processor.*": {"queue": "llm"},
+        "app.workers.llm_batch_processor.*": {"queue": "llm"},
         "app.workers.news_fetcher.*": {"queue": "news"},
         "app.workers.trend_scorer.*": {"queue": "news"},
         "app.workers.member_interest.*": {"queue": "news"},
@@ -90,5 +92,9 @@ celery_app.conf.update(
             "task": "app.workers.notification_dispatcher.send_weekly_digest",
             "schedule": crontab(hour=8, minute=30, day_of_week=1),  # Monday 8:30 AM UTC
         },
+        "poll-llm-batch-results": {
+            "task": "app.workers.llm_batch_processor.poll_llm_batch_results",
+            "schedule": crontab(minute="*/30"),
+        },
     },
 )