refactor: 移除 Python 后端源码，全面迁移至 Rust gateway 架构

- 删除全部 Python 源码 (src/) 及 Alembic 迁移脚本，归档至 _deprecated_py_src/
- 重构 Rust gateway ai_pipeline: 拆分 planner/finalize 模块，新增 contracts/adaptation 层
- 重组 handlers 模块为 admin/public/proxy/internal/shared 子模块结构
- 新增 executor 模块，引入 Rust 原生数据库迁移 (aether-data/migrations)
- 简化 CI/Docker 构建流程，移除 base image 二级构建，统一为单一 app image
- 移除 Python 相关基础设施文件 (entrypoint.sh, gunicorn_conf.py, Dockerfile.base)

apps/aether-gateway/Cargo.toml

@@ -42,6 +42,8 @@ sqlx.workspace = true
 thiserror.workspace = true
 tokio.workspace = true
 tokio-util.workspace = true
+tower = { version = "0.5", features = ["util"] }
+tower-http = { version = "0.6", features = ["fs", "compression-gzip", "set-header"] }
 tracing.workspace = true
 url.workspace = true
 uuid.workspace = true