feat: 添加种子数据和一键初始化脚本

- demo 账号: brand/agency/creator@demo.com
- 组织关系 + 项目/Brief + 4种阶段任务 + 规则数据 + 示例消息
- entrypoint.sh (Docker) + init_db.sh (手动) + start-dev.sh 更新

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/Dockerfile

@@ -40,6 +40,7 @@ COPY app/ ./app/
 COPY alembic/ ./alembic/
 COPY alembic.ini .
 COPY pyproject.toml .
+COPY scripts/ ./scripts/
 
 # 创建非 root 用户
 RUN groupadd -r miaosi && useradd -r -g miaosi -d /app -s /sbin/nologin miaosi \
@@ -53,4 +54,5 @@ EXPOSE 8000
 HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
     CMD curl -f http://localhost:8000/health || exit 1
 
+ENTRYPOINT ["./scripts/entrypoint.sh"]
 CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]