Persist chat history to Postgres via durable JetStream consumer

DATABASE_URL required at startup; schema self-initializes.
Compose gains a postgres service for standalone dev.

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

docker-compose.yml

@@ -8,10 +8,14 @@ services:
     env_file: .env
     environment:
       NATS_URL: nats://nats:4222
+      DATABASE_URL: postgres://cnats:cnats@postgres:5432/cnats
     ports:
       - "3000:3000"
     depends_on:
-      - nats
+      nats:
+        condition: service_started
+      postgres:
+        condition: service_healthy
 
   nats:
     image: nats:2.10-alpine
@@ -19,3 +23,22 @@ services:
     ports:
       - "4222:4222"   # client connections
       - "8222:8222"   # monitoring UI (http://localhost:8222)
+
+  postgres:
+    image: postgres:17-alpine
+    environment:
+      POSTGRES_USER: cnats
+      POSTGRES_PASSWORD: cnats
+      POSTGRES_DB: cnats
+    ports:
+      - "5432:5432"
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U cnats -d cnats"]
+      interval: 2s
+      timeout: 3s
+      retries: 15
+
+volumes:
+  pgdata: