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/executor/candidate_loop.rs

@@ -0,0 +1,132 @@
+use axum::body::Body;
+use axum::http::Response;
+
+use crate::gateway::ai_pipeline::planner::plan_builders::{
+    LocalStreamPlanAndReport, LocalSyncPlanAndReport,
+};
+use crate::gateway::scheduler::record_local_request_candidate_status;
+use crate::gateway::{
+    execute_execution_runtime_stream, execute_execution_runtime_sync, AppState,
+    GatewayControlDecision, GatewayError,
+};
+
+pub(crate) trait LocalPlanAndReport {
+    fn plan(&self) -> &aether_contracts::ExecutionPlan;
+
+    fn report_kind(&self) -> Option<String>;
+
+    fn report_context(&self) -> Option<serde_json::Value>;
+}
+
+impl LocalPlanAndReport for LocalSyncPlanAndReport {
+    fn plan(&self) -> &aether_contracts::ExecutionPlan {
+        &self.plan
+    }
+
+    fn report_kind(&self) -> Option<String> {
+        self.report_kind.clone()
+    }
+
+    fn report_context(&self) -> Option<serde_json::Value> {
+        self.report_context.clone()
+    }
+}
+
+impl LocalPlanAndReport for LocalStreamPlanAndReport {
+    fn plan(&self) -> &aether_contracts::ExecutionPlan {
+        &self.plan
+    }
+
+    fn report_kind(&self) -> Option<String> {
+        self.report_kind.clone()
+    }
+
+    fn report_context(&self) -> Option<serde_json::Value> {
+        self.report_context.clone()
+    }
+}
+
+pub(crate) async fn execute_sync_plan_and_reports<T>(
+    state: &AppState,
+    parts: &http::request::Parts,
+    trace_id: &str,
+    decision: &GatewayControlDecision,
+    plan_kind: &str,
+    plan_and_reports: Vec<T>,
+) -> Result<Option<Response<Body>>, GatewayError>
+where
+    T: LocalPlanAndReport,
+{
+    let mut remaining = plan_and_reports.into_iter();
+    while let Some(plan_and_report) = remaining.next() {
+        if let Some(response) = execute_execution_runtime_sync(
+            state,
+            parts.uri.path(),
+            plan_and_report.plan().clone(),
+            trace_id,
+            decision,
+            plan_kind,
+            plan_and_report.report_kind(),
+            plan_and_report.report_context(),
+        )
+        .await?
+        {
+            mark_unused_local_candidates(state, remaining.collect()).await;
+            return Ok(Some(response));
+        }
+    }
+
+    Ok(None)
+}
+
+pub(crate) async fn execute_stream_plan_and_reports<T>(
+    state: &AppState,
+    trace_id: &str,
+    decision: &GatewayControlDecision,
+    plan_kind: &str,
+    plan_and_reports: Vec<T>,
+) -> Result<Option<Response<Body>>, GatewayError>
+where
+    T: LocalPlanAndReport,
+{
+    let mut remaining = plan_and_reports.into_iter();
+    while let Some(plan_and_report) = remaining.next() {
+        if let Some(response) = execute_execution_runtime_stream(
+            state,
+            plan_and_report.plan().clone(),
+            trace_id,
+            decision,
+            plan_kind,
+            plan_and_report.report_kind(),
+            plan_and_report.report_context(),
+        )
+        .await?
+        {
+            mark_unused_local_candidates(state, remaining.collect()).await;
+            return Ok(Some(response));
+        }
+    }
+
+    Ok(None)
+}
+
+pub(crate) async fn mark_unused_local_candidates<T>(state: &AppState, remaining: Vec<T>)
+where
+    T: LocalPlanAndReport,
+{
+    for plan_and_report in remaining {
+        record_local_request_candidate_status(
+            state,
+            plan_and_report.plan(),
+            plan_and_report.report_context().as_ref(),
+            aether_data::repository::candidates::RequestCandidateStatus::Unused,
+            None,
+            None,
+            None,
+            None,
+            None,
+            None,
+        )
+        .await;
+    }
+}