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/src/scheduler/mod.rs

@@ -1,6 +1,7 @@
 mod candidate;
 mod failover;
 mod health;
+mod request_candidates;
 mod route;
 
 pub(crate) use candidate::{
@@ -21,6 +22,10 @@ pub(crate) use health::{
     is_provider_key_circuit_open, provider_key_health_score, provider_key_rpm_allows_request_since,
     PROVIDER_KEY_RPM_WINDOW_SECS,
 };
+pub(crate) use request_candidates::{
+    current_unix_secs, ensure_execution_request_candidate_slot, execution_error_details,
+    record_local_request_candidate_status, record_report_request_candidate_status,
+};
 pub(crate) use route::{
     is_matching_stream_request, resolve_execution_runtime_stream_plan_kind,
     resolve_execution_runtime_sync_plan_kind, supports_stream_scheduler_decision_kind,