feat(phase8.1-8.2): dynamic worker concurrency via worker_configs

- Migration 054: worker_configs table (queue_name PK, max/min_concurrency,
  enabled, updated_at); seeds step_processing(8/2), thumbnail_rendering(1/1),
  ai_validation(4/1)
- WorkerConfig SQLAlchemy model
- apply_worker_concurrency beat task: reads enabled configs, broadcasts
  pool_grow to all Celery workers every 5min
- GET/PUT /api/worker/configs (admin): list + update per-queue concurrency
- docker-compose.yml: worker uses --autoscale=${MAX_CONCURRENCY:-8},${MIN_CONCURRENCY:-2};
  render-worker uses --autoscale=1,1 --concurrency=1
- WorkerManagement.tsx: "Concurrency Settings" section with +/- steppers
  and Save button per queue

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

backend/app/models/worker_config.py

@@ -0,0 +1,14 @@
+from datetime import datetime
+from sqlalchemy import String, Integer, Boolean, DateTime
+from sqlalchemy.orm import Mapped, mapped_column
+from app.database import Base
+
+
+class WorkerConfig(Base):
+    __tablename__ = "worker_configs"
+
+    queue_name: Mapped[str] = mapped_column(String(100), primary_key=True)
+    max_concurrency: Mapped[int] = mapped_column(Integer, nullable=False, default=8)
+    min_concurrency: Mapped[int] = mapped_column(Integer, nullable=False, default=2)
+    enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
+    updated_at: Mapped[datetime] = mapped_column(DateTime, nullable=False, default=datetime.utcnow)