feat(ledger): add append-only double-entry engine

The Lightning bridge is modelled as the boundary with the outside
world and is the one account permitted to go negative; its negative
balance is exactly what is owed to players inside the system. All
other accounts are floored at zero by both the application and a
database trigger.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,31 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: arcade
+      POSTGRES_PASSWORD: arcade_dev
+      POSTGRES_DB: arcade
+    ports: ["5432:5432"]
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+      - ./migrations:/docker-entrypoint-initdb.d:ro
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U arcade"]
+      interval: 2s
+      timeout: 3s
+      retries: 20
+
+  redis:
+    image: redis:7-alpine
+    ports: ["6379:6379"]
+    volumes:
+      - redisdata:/data
+    healthcheck:
+      test: ["CMD", "redis-cli", "ping"]
+      interval: 2s
+      timeout: 3s
+      retries: 20
+
+volumes:
+  pgdata:
+  redisdata: