feat(docker): add postgres service to dev compose

新增 postgres:16-alpine service + named volume + healthcheck，作为 Stage 0+
默认 DB backend。生产可通过 DATABASE_URL 指向远程 RDS 时跳过此 service。
端口 / 用户 / 密码 / 库名都走 env var 覆盖（POSTGRES_USER/PORT/...
默认 deerflow/5432/deerflow_dev）。

YAML 通过 `docker compose config` 校验；live healthcheck 待 docker daemon 起后由
T1.3 testcontainers 路径覆盖。

Stage 0 PR1 T1.2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docker/docker-compose-dev.yaml

@@ -2,6 +2,7 @@
 # Usage: docker-compose -f docker-compose-dev.yaml up --build
 #
 # Services:
+#   - postgres: PostgreSQL 16 (port 5432) — Stage 0+ default DB backend
 #   - nginx: Reverse proxy (port 2026)
 #   - frontend: Frontend Next.js dev server (port 3000)
 #   - gateway: Backend Gateway API + agent runtime (port 8001)
@@ -13,6 +14,30 @@
 # Access: http://localhost:2026
 
 services:
+  # ── Database (Stage 0+ default backend) ────────────────────────────────
+  # Postgres for local dev. Production may point DATABASE_URL at a remote RDS;
+  # this service can then be omitted via `docker compose --profile <other>`.
+  postgres:
+    image: postgres:16-alpine
+    container_name: deer-flow-postgres
+    environment:
+      POSTGRES_USER: ${POSTGRES_USER:-deerflow}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-deerflow_dev}
+      POSTGRES_DB: ${POSTGRES_DB:-deerflow}
+    ports:
+      - "${POSTGRES_PORT:-5432}:5432"
+    volumes:
+      - postgres-data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER:-deerflow} -d ${POSTGRES_DB:-deerflow}"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+      start_period: 10s
+    networks:
+      - deer-flow-dev
+    restart: unless-stopped
+
   # ── Sandbox Provisioner ────────────────────────────────────────────────
   # Manages per-sandbox Pod + Service lifecycle in the host Kubernetes
   # cluster via the K8s API.
@@ -178,6 +203,8 @@ volumes:
   # image build are not shadowed by the host backend/ directory mount.
   gateway-venv:
   gateway-uv-cache:
+  # Persist Postgres data across container restarts.
+  postgres-data:
 
 networks:
   deer-flow-dev: