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/alembic/versions/054_worker_configs.py

@@ -0,0 +1,36 @@
+"""Add worker_configs table for dynamic concurrency settings.
+
+Revision ID: 054
+Revises: 053
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "054"
+down_revision = "053"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "worker_configs",
+        sa.Column("queue_name", sa.String(100), primary_key=True),
+        sa.Column("max_concurrency", sa.Integer, nullable=False, server_default="8"),
+        sa.Column("min_concurrency", sa.Integer, nullable=False, server_default="2"),
+        sa.Column("enabled", sa.Boolean, nullable=False, server_default="true"),
+        sa.Column("updated_at", sa.DateTime, nullable=False, server_default=sa.text("now()")),
+    )
+    # Seed default rows
+    op.execute("""
+        INSERT INTO worker_configs (queue_name, max_concurrency, min_concurrency, enabled)
+        VALUES
+            ('step_processing', 8, 2, true),
+            ('thumbnail_rendering', 1, 1, true),
+            ('ai_validation', 4, 1, true)
+        ON CONFLICT DO NOTHING
+    """)
+
+
+def downgrade() -> None:
+    op.drop_table("worker_configs")