feat(env): activate DATABASE_URL with local-dev default

.env.example DATABASE_URL 行从注释改为活值（指向 docker compose 起的本地
postgres）。同时加 RDS 示例注释作为参考。psycopg2 风格 URL 升级到
postgresql+asyncpg:// 与 SQLAlchemy 异步 dialect 对齐。

Stage 0 PR2 T2.2.

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

.env.example

@@ -38,8 +38,11 @@ INFOQUEST_API_KEY=your-infoquest-api-key
 # GitHub API Token
 # GITHUB_TOKEN=your-github-token
 
-# Database (only needed when config.yaml has database.backend: postgres)
-# DATABASE_URL=postgresql://deerflow:password@localhost:5432/deerflow
+# Database (Stage 0+ default; required when config.yaml has database.backend: postgres)
+# Local dev — start with: docker compose -f docker/docker-compose-dev.yaml up -d postgres
+DATABASE_URL=postgresql+asyncpg://deerflow:deerflow_dev@localhost:5432/deerflow
+# Remote RDS example:
+# DATABASE_URL=postgresql+asyncpg://USER:PASSWORD@HOST:5432/DBNAME
 #
 # WECOM_BOT_ID=your-wecom-bot-id
 # WECOM_BOT_SECRET=your-wecom-bot-secret