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)

crates/aether-data/Cargo.toml

@@ -15,7 +15,7 @@ futures-util.workspace = true
 redis.workspace = true
 serde.workspace = true
 serde_json.workspace = true
-sqlx.workspace = true
+sqlx = { workspace = true, features = ["migrate", "macros"] }
 thiserror.workspace = true
 tokio.workspace = true
 url.workspace = true

crates/aether-data/migrations/20260403000000_baseline.sql

@@ -0,0 +1,4 @@
+-- Baseline migration: marks the existing schema (created by alembic) as ready.
+-- All tables already exist from the Python era; this file is the starting point
+-- for all future Rust-managed migrations.
+SELECT 1;

crates/aether-data/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod backends;
 mod config;
 mod error;
+pub mod migrate;
 pub mod postgres;
 pub mod redis;
 pub mod repository;

crates/aether-data/src/migrate.rs

@@ -0,0 +1,6 @@
+use sqlx::PgPool;
+
+/// Run all pending migrations embedded at compile time from `migrations/`.
+pub async fn run_migrations(pool: &PgPool) -> Result<(), sqlx::migrate::MigrateError> {
+    sqlx::migrate!("./migrations").run(pool).await
+}