feat(docker): gateway depends_on postgres healthcheck (dev only)

dev compose gateway 段加 depends_on: { postgres: { condition: service_healthy } }，
确保 postgres 通过 pg_isready 后 gateway 才启 uvicorn。

prod compose 不动：production 用远程 RDS（不在 compose 内），无 postgres
service 可 depend；docker-compose.yaml 不加此 depends_on。

YAML 通过 docker compose config 校验。

Stage 0 PR2 T2.5.

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

docker/docker-compose-dev.yaml

@@ -150,6 +150,9 @@ services:
         UV_IMAGE: ${UV_IMAGE:-ghcr.io/astral-sh/uv:0.7.20}
         UV_INDEX_URL: ${UV_INDEX_URL:-https://pypi.org/simple}
     container_name: deer-flow-gateway
+    depends_on:
+      postgres:
+        condition: service_healthy
     command: sh -c "{ cd backend && (uv sync || (echo '[startup] uv sync failed; recreating .venv and retrying once' && uv venv --allow-existing .venv && uv sync)) && PYTHONPATH=. uv run uvicorn app.gateway.app:app --host 0.0.0.0 --port 8001 --reload --reload-include='*.yaml .env'; } > /app/logs/gateway.log 2>&1"
     volumes:
       - ../backend/:/app/backend/